copied this from another forum:
"International Standard ISO 5775-2:1996, Bicycle tyres and rims — Part 2: Rims: document:
"ISO 5775-2 defines designations for bicycle rims. It distinguishes between
Straight-side (SS) rims
Crotchet-type (C) rims
Hooked-bead (HB) rims
Rims are designated by their nominal rim diameter and their nominal width, separated by a cross (× ). Both are measured in millimeters. The rim type codes SS or HB precede the rim designation, whereas code C is appended to the nominal width."
some examples would be: SS 400×20, HB 422, 700x23c, the "C" designating a Crotchet rim.
Interesting, but it doesn't explain the usage where the 'c' is appended to designations for the tire size rather than rim size. And most clincher rims these days are designated as hooked-bead type rather than crochet-type (Sheldon was of the opinion that reference to 'crochet-type' was because that was the French term for hooked rather than any different rim design).
Still seems to me that the 'c' appended to bicycle tire widths is simply an error in notation.