Check the thread U.S. Customs Service 686 CS-1, I posted a pretty precise history of the CS-1 there. I'm not sure how yours got to France, possibly they were given to the French government as surplus, or someone exported one that was purchased here.

Prior to 1992, many surplus CS-1s were sold in exchange sales to distributors. The Customs Service would put large lots of surplus firearms out for bid in exchange for whatever new model firearms they needed. This saved the taxpayers a lot of money. When Bill Clinton became President, he stopped the exchange sale program. After that they were either given to friendly governments or destroyed.

The "M" marking means that is had been sent back to the U.S. Customs Firearms and Training Branch to be modified to correct a tolerance issue which would cause it to lock up. (See my post on the other thread.)