Just found this site, and quickly found this thread. Mine is a RFI with a 1947 date stamped on it. Been refinished and cleaned up- nice shape. Cost me $150 about 5 years ago.Had never seen one here in Central IL before, found it at a gun show; the seller, a handyman/gunsmith, said that he had seen a few before. Mine is chamberd for the 3" round, wouldn't have bought it if it hadn't been.
When I did some research back then, I found some reference to the original round, indicating that the original shells were simply .303 rounds fire-formed by shooting them in the bored out chamber, then reloading them with shot. Don't know if that is true or not, but it would make sense, especially after the war, when there would have probably been a lot of surplus ammo around.
Have never fire a rifle cartridge in it, but have been tempted to try it.