Thanks for the information. I must not have looked carefully enough, so I got out the OptiVisor.

I now see that there are three stamps below the four digit serial number. Two are proofmarks, followed by the number 68. They are repeated on the barrel adjacent to the receiver. So I guess its made in 1968. (First year of production, so thats a bonus)

Further down the barrel, on the right side just past the rear sight is also stamped the following:

0NM31/0-S+L+3

Any idea what this additional stamping might mean?