If you would remove the forend and lift the barrels from the action, and post full size close up pics of every mark on the flats and just forward of the flats we might be able to provide more information.
I would suggest squirting epoxy in the firing pin holes, cleaning up the external surfaces and hanging it on the wall, with the left side showing.

It was likely sold by a U.S. sporting goods or hardware store

Nouvelle page 0

 

Thanks!
Stay tuned...

 

Here's a bad photo showing some of the markings. I just figured out how to disassemble (thanks DrewHause!) and will give it an ultrafine steel wool and CLP cleaning later. Once that's done I'll be able to send a better photo.

 

Here's a bad photo showing some of the markings. I just figured out how to disassemble (thanks DrewHause!) and will give it an ultrafine steel wool and CLP cleaning later. Once that's done I'll be able to send a better photo. View attachment 147035

Belgian gunmarks