It appears that the mainspring is anchored to the underside of the upper tang. Although Browning's patent of 1878 also drops the breechblock and cocks the hammer when the lever is dropped, it has the mainspring in a different place. The Hopkins and Allen falling block, which resembles the OP's rifle, also has the mainspring elsewhere. So Browning and H&A are out, as others have opined.
The late Frank de Haas's book, "Single Shot Rifles and Actions," has meticulous descriptions of 55 different single shot rifles with excellent drawings of the actions. The Remington No. 2 and No. 4 both have the mainspring anchored to the underside of the upper tang, but they, of course, are rolling blocks and are out. I find no rifle with a falling block and this kind of mainspring mounting in the de Haas book. I would agree with those who think it is a one-off, probably done by a minor or amateur gunsmith. This person probably used a barrel from another gun, and filled existing dovetails with brass when he affixed the barrel to this receiver.
The de Haas book, by the way, appeared in 1969 and is an excellent reference for SS rifles, probably the best. I got mine used at a gun show some years back. It is a good idea to be on the lookout for one if you are interested in this kind of ordnance.