Old Colonel hopefully will chime in as he picked up a beauty of a French 24 last year. In the meantime, photos of it would be nice to see. Gil

PS: If for some reason the gun is neither fish nor fowl, RMC will make custom brass hulls to fit the gun. However, I suspect you will get an answer regarding bore size markings from a 24 ga. owner. Kirby Hoyt of Vintage Doubles may be of some help but he is not a board participant. He was at the Shot Show last week but should be back this week should you choose to call him.