The only modern such attempt of which I'm aware was Navy Arms commissioning a run of .32 and .41 calibre rimfire ammunition in the '90s, I believe. Despite the plethora of firearms chambered for these rounds, the effort was not sustained.
I pounced upon several boxes of these at the time even though I had no guns in which to use them. Glad I did. I've now acquired and fired (to the edification of folks at my local shooting range) a Moore patent infringement .32 revolver and a S&W 1 1/2 seven-shooter, both of Civil War vintage. I've also acquired an over-under Remington derringer and a single shot Colt Thuer, both .41 rimfire and both sometimes carried by me as secondary CCWs. These Navy Arms rimfire cartridges now go for as much as $7.00 a cartridge and I have an eccentric but handy friend who reloads them(!)
Am not sure there are thousands of Spencer owners out there ravening to acquire rimfire ammunition for their weapons, especially given readily reloadable alternative centerfire options are available which require no permanent modification of the weapon. I shoot an original cranky 56-56 and much smoother 56-50 with the drop-in centerfire blocks and flat-nose magazine followers. Were rimfire alternatives available, the economics of it, after a few range sessions, would keep me reloading centerfire brass.