That's a very nice rifle! My very similar one, serial no. 25xxx dating from 1938 has factory-fitted scope mountings of quite a different pattern. The mounts are also see-through, but not quite as high as Don's. They work OK standing, but not prone, with the pronounced drop in the stock.

But they are a brilliant design, with a great magazine, and an effective cartridge. 140 grain bullets give more velocity, but need to be seated out a long way to feed smoothly. Let's hope he finds a good new home!