Odd thing is that a regular 75 panel (2000 and 2003, MGR) works just fine in my R75V8 and vice versa. Was playing around swapping them a few years back to see if that improved things, which it didn't, of course. Part number is identical on all of them, so there is something odd about the 'after-market' panels (MG7?) only perhaps. Differences in the HVAC design between 260 and RV8 are well known of course, as well as the obvious major differences between V8/non-V8 of both variants. Looks like that stretches to other HVAC electronics bits-and-pieces rather than the panel and its packaged electronics, from what you say Mike.
David