That's the first generation of the Model 3, which we developed at Bianchi to compete with the Seventrees and the Sparks. "Advanced IWBs" were quite new in the early '70s and legend has it that Cooper popularised the style. Plausible.
The first generation Model 3 has a fixed belt loop integrated into the band stitched around the mouth of the holster. Quite soon afterwards we added (and patented) an adjustable angle version, wherein the loop is a separate part that is attached, rotatably, to the band. Yours has had the muzzle cut off
It was Mas Ayoob who got us to add the thumb snap, about which he still writes today.
The Model 3 preceded the Askins Avenger, which itself preceded the Sparks version by ten years (it is a myth that Bruce Nelson's was the original); 1977/8 vs 1988.