From what I can read it is a set of rifled 20ga Belgium o/u barrels made in 1954. Looks like it might have been built to hunt boar with slugs.

Here is the Belgium proof mark guide.

https://www.hunting.be/wp-content/uploads/2020/02/Belgian-Proof-Marks.pdf

As far as what they fit, is there no writing on the tops of the barrels. You might want to take your question over to the double gun site.

https://www.doublegunshop.com/forums/ubbthreads.php

Last edited by erich; 08/11/21.
After the first shot the rest are just noise.

Make mine a Minaska

Heaven has walls and rules, H-ll has open borders