While I had my micrometer out, I decided to measure the metal rods of the Wolff’s lead gauge, pictured above. As there is light surface corrosion to some of the rods, I took several readings from each and aimed for an accuracy no greater than ±0.0005 inch. The millimetre readings are conversions.
H: 0.0300" / 0.762 mm
2: 0.0355" / 0.902 mm
M: 0.0410" / 1.041 mm
4: 0.0420" / 1.067 mm
5: 0.0455" / 1.156 mm
6: 0.0490" / 1.245 mm
7: 0.0570" / 1.448 mm
VS: 0.0580" / 1.473 mm
P: 0.0610" / 1.549 mm
W: 0.0790" / 2.007 mm
A couple of observations:
W appears to be equivalent to Faber’s 2 mm leads, the main difference being that refills for 19th century British pencils tended to be much shorter. I measured some modern Staedtler 2 mm refills and these came out in the range 1.985 mm to 2.005 mm (average 1.995 mm).
There is no obvious 1.18 mm lead (3/64") - it would lie almost halfway between 5 and 6 - which supports the idea that this size was a later introduction, possibly by Yard-o-led and based on the standard American wire gauge.
H and 2 size leads were essentially the Victorian equivalents of 0.7 mm and 0.9 mm, which seems remarkable in the days before polymer leads. Obviously the leads were much shorter, and sometimes even came packaged in individual glass tubes, but in the 1820s Mordan sold an even smaller VH lead, apparently little used (even Mordan’s advertisements described it as “very small in size, seldom required”!)