IDENTIFICACION

[Dmitry]

Originally Posted by corrado26

The "U" under a crown is the German blackpowder proofmark used since 1891 until 1939. 2gNGP = 2g Neues Gewehr Pulver = New Gun Powder

If Neues refers to smokeless gunpowder, then 2 grams is a substantial amount.
2 grams is 31 grains. For reference, a 9 mm Luger round takes 5 grains of smokeless powder.
I am still curious about this revolver. There are no sights on any of these examples. There appears to be no provision for mounting it on a stock or stabilizing it somehow. The trigger is for distance pulling. A possibility is a burglar alarm, triggered by opening of doors or windows. 20 discharges is a lot though, for any burglar alarm.
If this was, as HH describes it, a gun for some kind of naval theatrics, it would've been designed to look like a cannon, not a giant revolver.