Not that it would bring clarity to the issue at hand, but nevertheless: Lots of interesting information here!
So the shotgun tube set appears to have nitro proof as well.
The left barrel definitely is choked, being marked "nicht für Kugel".
But both barrels show those lines or striations or maybe remnants of straight rifling.
So straight rifling plus choke for the left barrel??

A comment to Mike's remark of "hundred years of use with black powder with subsequent cleaning":
yes, that gun appears to have been used lot, but possibly not for 100 years straight, besides 24 gauge black powder shells not being available since WW2.
That gun might also have seen smokeless powder with corrosive primers and not enough cleaning.
fuhrmann