Mike I have not seen an M73B1 with a serial number that low before. The best information available IMO (Campbell, 2003) indicates the earliest scopes were commercial scopes either 330C's with cross wire reticles or 330 Scope - M.8's with tapered posts. In other words the really early scopes would not have had the military nomenclature electo-penciled or stamped.
For Weavers part both the 330C (click adjustable) and 330S(screw adjustable) commercial scopes were catalogued with cross wires or posts.
Considering the ease with which a 330C can be turned into an "m73B1" i would proceed with caution.
Regards,
Jim