... There is a difference. It did seem like a PIA to install. I wonder if I should press out the old stem and use it in the new mount?...
I don't think that would work (plus you did get it installed)
I'm guessing the ATX mount is matched with the ATX stem and the MTX mount is matched with the MTX stem.
The problem arose when mounts were ordered and didn't come with the stem... Then they wouldn't match.
(plus.. apparently pressing the stem in IS a PITA)
Sounds like yours was just hard to install but the guy in the picture had his whole engine pulled cockeyed.