I am beginning to believe that this may be a gun made by Jules Bury and then engraved by Ugartechea. That might explain why there are both Belgium and Spanish Proof marks. Or the other more likely possibility is that it is an Ugartechea copy of Bury. But then why the Belgium proof marks? In either case, I have not seen any self-openers made by Bury of Ugartechea made in this style... Did Bury make a self-opener?

Nonetheless, look at the locks carefully:

Bury locks:



These locks:


David