Thanks for the message about C4 in K code, reassuring as so many people already expressed that it wasn't produced.
On document when selled from US, it was described like this : "according to the Barrett_Jackson and Kruse auctions, only 6 known K codes were ordered with the C4 : 2 fastbacks, 2 convertibles, 2 coupes". I don't have any idea what this relate to (a month, a year, currently registered somewhere, ...).
For Mustang club in France, I guess this one is serious (born in 1984) due to service provided : http://www.mustangclubdefrance.com/index.php
Enjoy