It looks like the gun was made in the late 60's-early 70's, and is an R10. I presume that the 6A year designator continues, i.e 6B, 6C, etc, since 1975 the serial numbers begin with 7A. Under this premise, the one you've pictured would probably be from 1971-1973 perhaps.

SERIAL NUMBER FOR AFFIXED BARREL MODEL R V P & HS (Many Exceptions Occur)

6A...1968-1974

Cameron Hughes