Re: 41 cal rimfire
The ammo does turn up from time to time and companies used to turn some out every 10 years or so, but I haven't seen any new stuff lately as the ammo companies have had no "break" in which to produce odd-ball ammo.
I strongly recommend you check that Remington very carefully at the hinge; many are cracked. While the main cause of cracking was probably "flipping" the gun open, one with a cracked hinge should not be fired as the crack will only become worse. In fact, I recommend not firing that derringer at all; if you want a derringer to shoot, there are much newer and stronger guns. The point is not just "a collector's warning." A NIB Remington derringer with some markings could bring enough to buy a small car, so avoiding breakage is not just academic.
As a purely practical matter, the Remington, as you may know, has a mainspring a bit like one on a Ford truck. While they appear in the movies to be easy to use, they really are quite difficult.