Did you check out the proof marks and date codes against the resources that I provided in links above?
As to value, it's highly subjective, but think about it this way: a decent entry level target shotgun is going to cost you $2800 -$3000 in the US market. Are you in Germany? If so, add $1000. What we are looking at is a very, very good condition, high craft gun with above average engraving from a low volume gunmaker of some historical note, chambered quite usefully in 12ga (presumably 2.5" ?) and 9.3x72R, a useful mid-bore cartridge for deer and boar.
So, if you have the money and you want it, in my opinion (offered for free and therefore possibly of equivalent value), it is by no means overvalued. As to the utility of a drilling, that's another matter and only you can decide on it.