Hi, Back from a few days break...
1) There a few examples of KHK cases stamped with the C so that's not guys 1000 miles apart stamping the same thing at the same place on their cases ; - )
2) It it stood for close ratio set C, we could reasonably also expect the other 11 other prefered ratios for XLR and KR to appear sometimes??? (there's more, 48 combinations in all from the 20 factory gears...) the 12 are: KR, CLOSE, C, D, E, F, G, H, J, K, M and N... A few others were also named for the XRs in the 70s...
I still think the C to stand for close ratio box, but that's just my instinct...
Patrick