The O 50 had quite a long production run and a few changes along the way, some photos that show the stock (forestock esp.) would be a start.

There's a lot of interest in these guns (from me! Sadly & most unfairly, I'm not allowed to buy anymore "gun rubbish").

I cant remember exactly but the later ones may have a very, very tiny date stamp on the receiver above the rigger in a MM YY format.