I will attest to this much: the power supplies on the P2K/P1K modules. The very late ones appear to have better caps and are shielded. There are also some component changes on the main boards--might possible be different converters. I'm not sure how much, if any, audible difference there is.
On a purely superficial level, some Command Stations say "Made in the USA" while some others say "Made in the China".
I also wonder how the keyboard models (PK6 etc.) differ from the others.