Great definitions, I agree 100%. To my mind, that's what I believe this "first 505" is all about; they have identical hardware to all the ones that will follow, as @kazenorin points out (until V Mk II...!), with the only difference being this engraving to say the owner was epic enough to score one of the first run.
Something else is that 'Signature' implies exclusivity, while 'Exclusive' states it. Which might come down to a personal style thing (I prefer understated minimalism, but that's just me).
Personally I don't mind these either, though I do feel that the same point could be made that the hardware is still the same (i.e. the 'Second' edition and the 'Post-Launch' edition are the same as these engraved ones).
Hmmm... maybe it is "Edition" that is causing the problem? Maybe "Series" could help to get around our semantic issue?