your parallel with MESS makes no sense: the same arcade hardware can run different games in which buttons has different usage. on a keyboard, A is A, B is B, etc.
or are you suggesting we should label 2,4,6,8 on computer keypads as Down, Left, Right and Up because a lot of computer games use those keys to move the main character?
as with most software-related details, you should rely on external dats not on the MAME source because the hardware does not care how the buttons are used by the game...
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