Folks here are right on top of the one.

Juan Cruz Mugica, makers mark AB, frequently found with makers initials JCM in an oval. Makers crest on top of barrels. Proof year code G1, for 1961. Looks to me to be a model 427; middle of the road on price point. This gun has a Scott cross bolt, side clips and a reinforced frame. It has seen 55 years of use; treat it well and it will likely give another 55 years of service.

Outstanding gun.