May I suggest using the term 'physical compatibility' for that? When I think 'hardware compatibility' I tend to think of electronic devices working with other electronic devices.
I'll admit that hardware compatibility with the RC2014 backplane families is more interesting to me (than compatibility with any particular RC6502 design).To be clear, when I say 'hardware compatibility with the backplane' I just literally mean being able to use an existing RC2014 extension backplane to plug in more modules.
