I'm pretty sure that is not an M10 follower but who ever did that work was very good indeed. Check the top of the bolt body, about half way up for a rivet. This was a modification that was done to help make sure you do not re-assemble the bolt improperly. That is a great find and a lovely example of the craft at it's finest for the time. That "Old guy" must have been some tall to require that butt extension to be added as well as the recoil pad. Please let us know how the old girl shoots.
As a note, .300 Win Mag came out in 1963, so the work was obviously done sometime after that.
Scott