Nitro proof was available as an auxillary proof in 1900 Germany but not very often applied, due to the rules of proof creating a massive overload. I have a Sauer/VL&D not too far away in SN from this one. Mine was likely built for a waterfowler as it is about 7½ lbs with 30" tubes & chambered for 2 3/4" shells. Mine is a back action sidelock with a rather unique sear arrangement which seems very well thought out. It is by no means a "Best" gun, has some scroll engraving, but seems to be very well made & is still tight & on face. I think perhaps you either mis-read the 13/4 mark or it was misstamped. Both should be 13/1 which denoteds a bore size at time of proof large enough to accept a .719" gauge but not accepting a .729" one. There should be no 13/4 mark.
Miller
