Interesting. The MP-3 and MP-4 aren't 'scope cameras. Most of the lenses for the MP-3 were Rodenstocks, mainly Ysarons, most in, a few front-mounted on, Prontor Press shutters. I think they're all #1s, could be mistaken. Most of the lenses for the MP-4 were Tominons, all to be front-mounted on a #1 Copal Press shutter with no diaphragm. I don't think your unfortunate Oscillo-Paragon was sold to be used on either MP.
3" is ~ 75 mm, is the focal length of many of the Oscillo Paragons that come to market. f/1.9 is right for many of the three inchers. AFAIK they were all mounted in Ilex #3 shutters.
There were also 56 mm (2.20" according to Ilex) Oscillo Paragons, at f/1.2, f/1.9 and f/2.8. I have no idea which sizes of Ilex shutters they were mounted in. AFAIK, Ilex didn't buy shutters in, they used their own.
Lens cells that fit an Ilex #3 don't go in a Compur/Copal/Prontor #1. Too big. So your 3" focal length must be a typo.
The best information I have on Ilex shutters says that none is threaded like any Compur/Copal/Polaroid. There may be an exception, I have a tiny Ilex electronic shutter -- smaller than the cataloged #1 -- whose threading seems to match Compur/Prontor #00, but with a shorter tube length.
Please look at what you have and tell us again what it is. As for unscrewing what you screwed in, if you didn't use threadlocker look for a couple of strap wrenches.
