People do stupid stuff when they don't know much about milsurp firearms.
I inherited a S&W Victory chambered in 38 S&W and some ammo for it, that turned out to be a partial box of 380 ACP. I suspect that since it was a partial box that someone in the past had successfully fired the gun with that ammo.
I can imagine an old codger bringing his gun down to a local gun shop and then spending 30 minutes behind the counter stuffing various ammunition down the chamber until he found something that fit.
Scary, but I bet that stuff happened a lot back in the day.
This I can see happening, especially if the previous person shot old British 38s&w ammo, it would have said 380 inch on the box