I hope some of them have some input.  Most of what I have discovered is thanks to Michael Petrov.  Tomorrow, we're going to disassemble it again, see if there are interrupted threads in the receiver.  There isn't on the barrel stub, and I didn't think to look in the receiver.  If there are no interrupted threads, then Niedner converted a solid frame to coil spring by his own means.  I think it is a regular Hornet, no other cartridge has the base diameter I measured and the owner found a fired 22 Hornet that wouldn't fit, probably because it was fired in a more liberal chamber.  I got him some new PRVI Hornet Ammo that ought to.  The bore was dusty, my calipers measured a .219 bore diameter.