It all started back in the day because folks didn’t want to remember or type out transformer codes, so it was easier to say “Mk III transformers” because that specific transformer happened to be used at one point on late Mk IIIs. I’m as guilty (more guilty?) of saying it over time too, because I’m nothing if not lazy while typing
It got murky over time because the internet theorized - as it does - that those specific transformers were sonically superior and a reason why older Rectos sounded different than Gs (it’s not).
The 561140 is a better and safer transformer to use in 2 channel Rectos, specifically on spongy and with rectifier tubes. That’s why Mesa transitioned to them. Same with the Mk IV that had a tweed power option.
