I don't think it is either, but the "sniper pistol" bit is amusing.
I am not sure, but I think there may be some means of attaching a stock, maybe a threaded hole in the bottom of the pistol grip. That looks like one of the general class of guns known as a "pocket rifle". They were used by poachers, but also just by people who wanted to take a walk in the woods and do some plinking or maybe get a rabbit for dinner. Stevens made much the same kind of single shot .22 in this country. They could be carried in the deep pocket of an overcoat, then assembled as the need arose.
If there is a means of attaching a stock, the stock may well have been discarded because of the U.S. federal law controlling pistols with shoulder stocks (short barrel rifles).
The proofs are German, and long antedate East Germany; collectors call them the "bug" marks for obvious reasons. The "B" indicates that the gun was proofed ("proved") in finished condition; the "U" is the second proof, required of all guns; the "G" is the proof for rifled barrels. Those marks were superceded by the 1939 law, so the gun was made before that; going by the general appearance, it was probably made before World War One.
D.R.G.M. means Deutsches Reich Gebrauchsmuster, or "registered design", an indication of protection for a design that did not meet the criteria for a full patent.
Jim