My limited experience matches Larry. All the post-WWI guns I've seen and hunted with have had proof marks. I have a buddy who runs a gunshop in Brittany and I've used a bunch of his SxSs for hunting woodcock over the years and as far as I can remember I've seen proofmarks on the guns (probably a sample size of 30-50 guns or so). Having said that I've much less experience with 19th century French shotguns.