It sounds like you really got lucky. Your story is the kind of thing that collectors dream of.
I suspect that few A4's were stolen from the military. They were too long to fit in a duffel bag, & GI's were unlikely to have the screwdrivers needed to take one apart. Pistols were a lot easier to hide......
A small quantities of A4's were sold by DCM in the early 60's. Quantities were too small to have an advertised sale, so they went to competitive shooters & members of the military who were "in the know". Their existence was unknown to collectors at the time, & many were sporterized.
Several hundred were sold by CMPin a lottery in 1999. Condition A rifles came with the scope base installed, Condition B rifles came without a scope base. The left windage adjustment screw may be staked, so remove only the right screw to install your scope.
You might want to post photos of your rifle for us. Whether it is "unissued" or "unissued since rebuild" might give you more choices in your scope selection.
Bruce Canfield (August 1999 American Rifleman Q&A column) believes that the M8 scope was produced only for a military contract, after the M73B1 scopes began to be delivered.
Neal