Thanks KCW! I had despaired of finding a reasonable priced alternative so I have just been using the original scope but now perhaps I can put on a Leupold!
And now that you mentioned it, it does seem more likely/logical that a new barrel was installed with the 1968 proofs. I am certain that I didn't have the utter and complete history of a rifle that was in someones possession for nearly 40 years; a lot of could have happened to it beyond the records I have.