Proofmark number 9 looks to be most similar which says that it's reserved for muzzle loading pistols since 1925.
I hadn't gone down the whole list myself.
At the bottom of the page is a list of date codes.
If the date code is an "O", the proof year is 1976.
If the date code is an "M", the proof year is 1974.
http://damascus-barrels.com/Belgian_All_Proofmarks.html