Sounds like an interesting gun.
Pictures make a big difference, especially pictures of the proof marks on the under side of the barrels.
Based on what you have said:
FW Kessler was the maker in Suhl Germany.
I bet the gun was made in the 20's or 30's
For info on the 8x57 check out this link
http://www.gboreloaded.com/forums/index.php?topic=128439.0Does the gun have a scope and or claw mounts?
Do you know if it has been fired?
The 16 ga are going to be 2 9/16 with full chokes unless someone worked on the gun after in was brought back to the USA. The modern 2 3/4 shotgun shells will cause problems without the chokes being opens and the chambers being drilled out for the 2 3/4 ammo. I understand the 2 3/4 shells will fit into the 2 9/16 chambers but the 2 3/4 shells should not be fired from the 2 9/16 chamber.
Also check the underside of the barrels for nn/nn (eg 10/29) which will give the month and year of manufacturer.
Let us know when you get the pictures...