It is never a bad idea to be cautious with an unfamiliar weapon. But the examination you did pre-firing, particularly of the barrels, sounds like an adequate one for a 1960's era gun, and Zabala made good barrels. I've shot several. I certainly wouldn't worry about a barrel burst.
The wrap against this era of Spanish gun is some of them had inadequately hardened internal parts... triggers, sears, safeties, springs and hammers sometimes wear prematurely. I don't know this to be the case for this Zabala. I haven't experienced it in mine.
A "poor man's" choke gauge is a dime. A dime won't fit into a "full" choke 12 ga. from the muzzle, but will fit into more open bores.
