My smallest mechanical pencil is not intended for writing but for use with a compass.
It’s just under 52 mm long, has a diameter of 6 mm at its thickest point and weighs about 5 grams.
This insert is from STAEDTLER and had the part number 956 0x (with x = 3, 5 or 7, depending on the lead size). Unfortunately it is no longer available. – Here it is in the STAEDTLER Mars professional 555 compass which is also discontinued.
Besides these inserts with a 4 mm spigot there was a 3.5 mm variant.
The twist mechanism makes it possible to insert it into a holder of some sort and become a usable mechanical pencil?
No, the twist mechanism is used for locking. The lead is advanced by pushing the slotted end (and the step below) into the tube (the marked part).
By the way, in Germany we had the STAEDTLER CIRCOFIX which, in addition to 0.3, 0.5 and 0.7 mm, was also available as a leadholder (2 mm). The CIRCOFIX was sold from the early 1980s to the mid-1990s.
I like that Staedtler Circofix! It looks a bit more “finished off” when removed, like a self-contained mini pencil not required to be docked to a mechanism.
Smallest pencil in terms of length (not width) in my collection has to be this curious Mitsubishi. I can’t find any model number on it.
Ah, compass inserts! Love them, but yet another fantastic rabbit hole… 
When it comes to compass accessories, here in Europe we had the fortune to enjoy not only the type of inserts available as well in Japan like the one described by @Gunther — the most comprehensive list of such exotica I could find was inside the monumental catalogue by Japanese company “DRAPAS” (or “Do-ra-pa-su”, as they probably call it), which showed specimens of at least two different types, by Staedtler Japan in 0.3, 0.5 and 0.7, and by DRAPAS itself for local style compasses — but also a wider range of plastic-body mini-pencils produced by Staedtler, Faber-Castell, rOtring and even other makers (Haff, Ecobra, Kern, Proebster…).
What I can recall on the spot goes as follows:
• Staedtler had an almost complete range of lead sizes (the Mars Circofix series lists 0.3, 0.5, 0.7 and 2.00 mm, blue-body inserts, with at least two different types of pushbuttons — conical and cylindrical — and different color codes), AND it offered an amazing plastic detachable handle which could be screwed on the inserts, thus transforming the mini-pencils into almost regular-length pencils. This handle came only in larger compass sets from the Staedtler Mars Technico lineup, and had a delightfully useful lateral window, allowing for an easy push of the rear button on the insert without the need to unscrew the handle itself — I consider this among the greatest inventions in drafting accessories history; really, you have to see it to appreciate its genius.
Also, Staedtler had a nice 0.3mm cylindrical metal insert, much more similar to @Gunther 's version, not designed to be used a standalone pencil, even if… (see end of this post). And a couple of metal inserts to be used in combination with drop-bow compasses designed to accommodate inking pen tips.
• Faber-Castell offered for sure similar 0.5 and 2.00 mm inserts (probably made by the same manufacturer, only in green color matching their color scheme), and I’ve seen one 0.3 mm insert, all available within their “Ultra” lineup. Unfortunately, to my knowledge, no handle was available.
• rOtring (and later Sanford) offered an equally wide range of inserts (listing 0.3, 0.5, 0.7 and 2.00 mm inserts, with a burgundy-ish color for the plastic bodies, and metal pushbuttons and tips), adding up to their already remarkable series of other inserts, such as the cutting-edge one to use a compass as a circle-cutting tool, or the pair to be aptly inserted in large compasses or drop bow compasses. Still, no handle available, at least as far as I know.
rOtring’s design was later used in all Haff’s compasses (or it goes the other way round: Haff was producing rOtring’s high-end compasses, rebranding their own with the unmistakable red circle), and heavily inspired what could be found in other compass sets, such as the ones by Ecobra (black body, round “knurled grip” section) and Kern (black body, hexagonal “grip” band).
Diving deeper into the rabbit hole, we get to some lesser-known manufacturers, most notably “Original Werein” (or “Werein Aristea”) in Italy: this company had full-metal, very tiny compass inserts with knock mechanisms for lead feed, intended to live only as compass attachments, and sold as accessories in their larger compass sets. Werein’s inserts came all in 0.5mm, but sported two different diameters for their rear attachment to the compass legs, in order to be used with both large compasses, and smaller bow compasses.
Then we get to the very weird stuff: Proebster in Germany (if I remember correctly) offered a very curious set containing only a slim metal pencil attachment — a standalone item, rather than an accessory in a large set — with ornamental grooves and a super-thin profile, which itself came with accessories, i.e.: a clearing pin with protecting metal sleeve; three boxes of extra-short lead cores (in H, 2H and 4H hardness); and a graceful handle like the one used in compass sets to attach the ruling pen head, to make a fully functional ruling pen.
So, Proebster did think that a compass attachment could indeed be used as a regular pencil, and provided the apt setup to ensure a proper use of its magnificent accessory. I ought to have all this material somewhere round here: it really deserves to be seen.
[Then we have the twistaction pencil inserts made by Kent/Uchida or similar Japanese companies, and included in Haff’s ellipsographs, but those are really unicorns, and I’ve only seen them on a blog post by Graphography and in a few frames in an old YT video explaing the use of the plastic ellipsographs…]
Do you know how rare these are? They piqued my interest long ago but I have yet to find one on eBay or similar sites…
Wow! @Leonov, thank you for these amazing details and for that exciting journey into the special world of drawing instruments and accessories!
Yes, the Circofix handle with its window was erally great (I forgot to mention it). I think that the upper thread on the inserts was intended for use with that handle (the lower fits into the holder of the compass).
I still know Haff from the time when they were making compasses and accessories (I must have an old catalogue somewhere). Sema-Seeling was another German maker but both have stopped making these instruments quite a while ago. – I have a Haff ellipsograph but unfortunately without the pencil.
Speaking of Graphography: Mechanical Pencil Month – Day 14: Compass inserts
Do you know the STAEDTLER Galileo compass?
Unfortunately I don’t know. I have some STAEDTER inserts from the late 1980s as well as their Mars technico 555 compass which was sold with a 0.5 mm insert (the later variant of that compass, the Mars professional 555 was sold without it). – There’s currently a technico 555 with a 2.0 insert on eBay: Vintage Staedtler Mars Masterbow 551 02 Mars Technico 555 02
Yes, definitely an interesting piece: I’ve seen a few in a drawer of a now-closed shop here in my city, but actually never bought one, because while the design was clearly intriguing — and I think it even won some prizes for its peculiar shape and mechanism — I did not see it as an effective instrument when it comes to actually working and drawing circles with high precision.
I’ve tried many different brands and solutions, but to this day nothing beats the high-end pieces by Riefler, Haff, and Kern (probably in that order, especially when it comes to regular pairs of compasses: bow compasses, drop-bow compasses and specialty items can be an area where the top ten varies a lot more).
[By the way: receiving a “thank you” by the one and only @Gunther from Lexikaliker, the guy who introduced me to the world of great pencil sharpeners and rare historical pieces from the grand German manufacturers and pencilmakers, is really making my day 
]
As for the inserts: I advice @stu-dying to keep an eye on local versions of Ebay (ebay.de, ebay.fr, etc.), as sometimes the inserts do come up: be prepared though, as in my experience it’s much easier to find rOtring’s inserts than Staedtlers’, and Faber-Castell’s ones are even rarer. It takes patience and a lot of work to put alerts on the keywords/queries, but every once in a while a full set comes up, and it’s usually at an affordable price.
Here’s a photo of the Mars Technico 555-40 compass set with the Circofix handle: STEADTLER Mars Technico 555-40 compass set with Circofix leadholders : 네이버 블로그
In the meantime, I managed to find all the pieces of the Proebster set — the box is buried somewhere, I’ll dig it up some other day.
This is the “disassembled” version, with all the different components shown separately (handle, compass insert, clearing pin with protective barrel, three lead containers with aptly shortened cores in three hardness levels):
In this second shot, besides the poor focus, you can see how the handle and attachment can be fit together to create a — very thin, admittedly, but still functioning — mechanical pencil which in principle can work with regular 0.5mm lead cores, albeit a bit shortened:
Hope it helps. Greetings!
