Greetings! Since you asked… To begin with, “Fixpencil” is not a brand, or at least I’d not consider it an entire brand, as the manufacturing company for all Fixpencils is Caran D’Ache, a well-known Swiss MP and art supplies maker, which also dips in the luxury sector with its high-end products.
The most famous Fixpencil is probably an all-metal, hexagonal-body, black aluminum leadholder, and it typically comes in 2.00 mm and 3.15 mm — both types can exhibit either a sanded grip area close to the tip, or a smooth finish on the whole barrel.
Then there is a 0.7mm MP Fixpencil, currently in production: same lightweight metal barrel, this time in different colour and with a glossy finish, but thin lead core. The Fixpencil has been rearranged as a ballpoint pen, and as such is currently sold in many variants and limited editions.
Other notable members of the family include a few older 0.5 mm MP’s (one with sliding sleeve, one with cushioning sleeve, one with fixed sleeve and extra-long barrel, discussed here The extra long Fixpencil… / on Knockology and in other threads within the Forum), an extra-short 2.00 mm leadholder “Fixpencil 23”, and an artist-oriented 4.4 mm leadholder with round body, the Fixpencil 44 “Museum”.
If you search around, there is a gallery on Flickr with many other rarities in the pencil series, including variants in the grip area (Fixpencil 884 “Junior” similar to the Conté-Gilbert-Blanzy MP’s), imprints and other apects of the overall pencil structure.