Look at the firing pin recess in the breech block. If converted, it will be centered toward the cartridge. If still rimfire, the recess will be off-center from bore (just like a .22). You may also see where a sleeve was installed after drilling. It appears this gun may have been rebored/relined/rechambered, considering the caliber stamp is polished and possibly restamped. (for the cases where these guns and similar ones such as the Stevens were converted, .38 Smith and Wesson, and or Special was a common choice).
This might have been accomplished near the same time as the sight "adornment".
If still .38 Rimfire, cartridges are actually still "engineered" at considerable expense, by a guy who manufactures his own brass and drills the head offcenter, installing a primer off-center. If you would just like one cartridge to have and show with you gun, I have a very few of the old ones on hand and might be persuaded to part with one.