The B1 on the flats is the date of manufacture---1956.
The F N is the mark of Arizaga.
20-65 indicates 20ga chambered for 65mm shells. From your measurements sounds like its chamber got increased at some point to 70mm (2 3/4 in). I would be surprised to see short chambers on a gun made for import to the US.

A friend has a 12ga sidelock Arizaga. Seems to be of medium quality. I think Arizaga didn't survive the DIARM fiasco of the early 80's.

Jeff