Yes you can shoot 2 1/2" RST copper #6s through this gun. It really isn't an old gun materials and construction wise. The marks on the barrel flats are pretty straight forward. The stylized eagles are the provisional proof of the raw barrels and the final proof of the same when finished. The crown/S indicates the gun was proofed as a smoothbore, the crown/W indicates choked barrels. Crown/U is the final inspection and proof acceptance mark. the circle with 16 inside indicates the shell size the gun was intended to use. The 16/1 says the bore diameter is actually a little tighter than 16ga which is pretty common on German guns. Since there is no shell length dimension stamped (eg 70mm) the chambers are by default 65mm (2 1/2"). 5/28 is month and year of manufacture or final proof. The eagle/nitro indicates the gun was proofed for use with smokeless powder loads (versus black powder). The five digit number on both barrels and the action is probably reference to proof records. The number on the action table, 10239, and the 39 deep in the action would indicate this is the serial number as it would have been put there prior to final assembly. Or it may be the proof house re serialed the whole thing as the barrels lacked the 10239. There are often little mysteries among the marks.
