Yeah, several people have asked where "U mount" originated. It's possible Cherry just copied it.
Back in these days, both Cherry and Alps were toying with various mounts, including cross and tee switches each. (The Cherry tee switches may take M7 keycaps; don't know.) There are other switches with crosses — the Futaba or Futaba clones on the BBC Micro look U mount, but the dimensions are slightly larger and Cherry caps won't fit.
Cherry settled on a cross with 1 mm by 1.5 mm arms. What cannot be proven from that one photo, is whether MEI are using a cross of those exact dimensions, or something annoyingly different.
It gets worse. The
Cherry M8 switch (which predates MX) has a cross with those dimensions, but M8 and MX caps are not interchangeable due to differing stem lengths, so, perhaps to assist companies with M8 tooling, there are MX switches with shorter M8 stems! This is in addition to the MX Orange (with a non-standard cross) and the MX switches with Z mount (Alps) stems ...... Without a vintage catalogue, I cannot prove that the Orange and Z mount variants are actually MX (i.e. MX* product codes), because if so, then the U mount categorisation at the wiki needs to be moved onto the individual switches that actually are U mount, not MX as a whole. (If not, then MX Orange isn't MX, just Orange :-)
The more work you do on the wiki, the more you realise is missing from it. So much knowledge scattered about in forum topics that needs to be collated. (If people each just covered one switch they're familiar with, we'd get there.)
In terms of MEI, it may be slightly different to 1 by 1.5 mm cross arms, or it may need special keycaps like the M8 did. I've e-mailed MEI for specs, but I don't know that it's even the same MEI. Probably not. Can't find another suitable MEI.