Yeah, Eli, and the Primo Jr. is a fixed lens TLR, not an interchangeable lens SLR. Not comparable at all in any sense. The Primo Jr. is a gem, Topcon's leaf-shutter SLRs are at best undistinguished. Every bit as sorry as, say, Contaflexes and Voigtlaender's SLRs.
About proprietary, well, good luck finding a T-mount lens or a TX- or a T4- or an Adaptall-mount that will work on a Uni. These lenses are in barrel, have their own diaphragms. Back when there many have been a little demand for foul alien unclean non-Topcon lenses for the Uni but there was no supply. As I said, good luck.
Basically no one made lenses in M-mount to be used on Leica M cameras. Not enough of 'em. There are now alternative M-mount bodies and alternative lenses for them, but this is a new development made possible, I believe and could be mistaken, by the expiration of Leica patents.
Paul, Exakta made few, if any, lenses. The focal plane shutter Topcon SLRs are in Exakta mount. There are zillions of old lenses, mainly German but some "third party" Japanese, that will fit them. Given how good Topcors are, if I had a Topcon reflex I'd stick with them unless I had a special need that no Topcor could meet.