Vince - The S&B 9 mm Br. C. headstamp most certainly is not a "grey" product. It is the standard headstamp of Sellier & Bellot .380 Auto (9 mm Browning Short) ammunition as exported all around the world, and has been for years. Not countring military headstamps and headstamps made for other companies, but rather only the Standard Sellier & Bellot-Brand commercial headstamp, the sequence of use is:
SBP 9 mm K (K = Kurz, German for "Short")
SBP 380 AUTO (I have only seen this on rounds in nickeled-brass case, made for sale to Mexico, and probably to some government agency there rather than commercially, since pistol sales are largely illegal in Mexico and have been for years.)
S&B 9 mm. K.
S&B 9 mm. C. (I have only seen this with the headstamp also having either one or two of the S&B Trademarks of an arrow pointing to a clean rifle bore, representing their Neroxin non-corrosive primers, and I have never seen the "K" designation WITH those other marks. That does not say that either or both does not exist, of course. I simply have never encountered them).
The final headstamp has been their standard head marking for years now.
Instead of asking the question of why they would use a "C" representing "short," I will ask simply "why not?" Aside from the fact that Sellier and Bellot has a French heritage, it also exports ammunition to France, Italy, Spain, and South America, all of which have a word for the English "short" beginning with a "C." Whole the USA is the major export market, I think, for commercial S&B pistol ammunition, it is not the only one and all the others combined may exceed that market. I just don't know. Also, French has been a court language and the language of the International Postal Service community for many, many years, so it is very much an "international" language.
I know, EOD, that is not much of a comprehensive answer, but then, perhaps there is no real comprehensive answer, but rather just a decision by a company of how to abbreviate the caliber, and in the overall scheme of producing and exporting ammunition, probably a decision considered to be of little importance.