There is a chart of common German proof marks at:
Some discussion and a chart of the strange German gauge markings for rifle barrels at:
I doubt there is any way to tell exactly when it was made unless post 1912 with a date code. There is no way to tell how many old Rob made, we dropped a lot of bombs on gun factories at one time, you may recall.
It is, of course, worth precisely what your friend paid for it... to him.
As to ammunition, bear in mind that it has 65mm = 2 9/16" shot chambers, you will need to scrounge up some correct shells instead of the US standard 2 3/4"