I recently purchased; I believe to be a French 16-gauge sxs gun. I've done a little research and noticed on the barrel flats there is no "65" marked. Does this mean it would not be a 2 1/2 shot shell size? I've been shooting 2 1/2 RST but would like to shoot some Boss 2 9/16 bismuth for hunting grouse on federal/state land.

thanks for any help on this.