I believe it's a Grulla. Grulla makes a gun called the Windsor. I have owned a pair in the past, a 12 and 20 made some years ago identical.
My brother owns several ones, a pair of 12 and 20 identical, a matched pair of 16's that were imported to England and a 12 bore also made under another name that was an European import.
Most were self openers but I believe one of his is not.
Grulla still makes the Windsor, look it up.

Mike Proctor