I have seen such similar item in the past. you should be aware that they are dangerous to even possess, much less use, and should be disposed of at your earliest opportunity.
since I am in a magnanimous mood, I will agree to take this highly hazardous item off your hand at no charge and dispose of it in that correct manner; send me a p.m. and I will supply you with the address...
seriously, you have something of enormous value: I believe that it really is a House rifle.
congratulations on a once- in- a- lifetime find!