Very nice gun! The only HAL I ever owned was not a diamond grade. Believe the experts here ID'd it as a 185 grade--the 185 being its cost long about WWI time. Lightest 2 1/2" 12 bore I've ever owned, at under 5 3/4#.

Steve Barnett has a 16ga beauty on his list.

Only thing of interest I can pick up from the proofmarks, which appear to be typical for that era, is that as a 13/1, your gun is slightly underbored. Not at all atypical for German guns.

If you have the proper screwdriver, you can probably time it correctly by removing it, adding a bit of blue Loctite, and putting it back in at the 12 o'clock position.