.. Perhaps the patent had run out? One of the exciting things about buying anything at auction, on line, is finding out what you actually get for your money when you take delivery.
Banjos are special cases in that some makers only put their marks somewhere in the pot and these are sometimes only revealed when taking off the resonator. You have paid the price hoping it was made by the maker you thought, and also, no one else knew better.
I bought a mixed lot recently that included a banjo-mandolin, in a nice case. I took little notice of it at the time but taking it out of the case revealed hardware on the back.
In these two pictures the technology in the un-named banjo-mandolin, almost certainly made in Birmingham in the 1920’s, (The neck clamp and the ridge in the heel are typical of Matthew) goes back at least to the SS Stewart some 30 years earlier?
In the case of the banjo-madolin (click on the picture) the neck tensioner goes through the pot rather than under it as in the case of the Stewart.