leatherwood 3x9 x40 variable with art- probably won't be EVER able to find one of those
however, you have to make a choice as far as the mount goes as well- mounts from that period were far inferior to those offered today- typically a single screw with matching "tongues" to match the grooves around the scope mounting hole in the receiver
and it should be made of steel- i would suggest unless you're stuck AND MUST HAVE
a nam version of the m21/m14 you get close to the dmr rifle of today as possible
i've got an m14 that i built according what i've read and most of the improvements at the time, and it wears a mount by s&k single screw- basically, what they did was take the national match version of the m14 and stick a scope and mount on it- which was envisioned way back when the rifle was designed-the reason for the single point mount was such that if the scope went south on you, you could dismount the works and use your stripper clip guide- ie it became just another m14- and they had lots of trouble with the scope and mount- and you have to crane your neck to use the scope- no cheekpieces back then, although i do remember a couple of guys filling a sock with sand and placing it atop the stock-
the modern mounts today do away with the stripper guide, unless you go super cheapo and then theyre aluminum, which exands differntly from steel, and loses your zero as it heats up-
you're probably trying build a norc 305 into a nam era 21, , so my best advise is to trick it out the way you want, but forget about doing a nam era build- parts are hard to find, it's uncomfortable and there are better versions today- both in terms of comfort and relaibilty