The old cartridge was a UN72 and it had the number 73 on it. I'd assume that means it's a 73mm.
When I read all the specs to the FIRST Jensen rep and told him was noobie said, his response was "That sounds about right." Not too convincing. When I had to call Jensen back because their website wouldn't complete my purchase, I asked a second rep and he gave me even less confidence. Shouldn't these guys know a little more about this?
Once I was assured the bracket was returnable, I ordered TWO, just so I'd have the "right" one next time I need to replace it. If I get stuck with two - PLUS the one that's not fitting - I'm giving up cycling forever and taking an Uber on the bike trails.
Also, YES, I have thoroughly cleaned the threads with a wire brush and I have backed off and re-tightened many, many , many times. Today I will try removing the cable-guide screw.