This showed up for sale here recently. There was a minor flurry of interest in such "tankers" some years ago which fortunately didn't last.
What make this one a bit interesting is the configuration of the receiver and the apparent bayonet lugs on the shortened barrel; which could be just a bored sleeve made from the end of a standard barrel and soldered on.
On the other hand, the photos seem to be arranged not to give too much away, and the seller is uncommunicative so no further info on markings etc.
Possibly something put together by someone at Long Branch from odds and ends; something we know happened fairly often.