Hello,
The PSR500, PSR500m and PSR85 are all the same exact keyboard with a few visual differences:
The 500 has grey, orange, yellow, black and purple buttons.
The 500m has orange, yellow, black and purple buttons, the digits for both rhythm and voice sections are also black. No grey buttons, this keyboard was only available from music stores, hence the 'M', and supposedly no grey buttons make it look more professional next to other higher-end equipment??? I fail to see the logic here but that was Yamaha in the 90's.
The PSR-85 has blue, red, grey, orange, green and yellow buttons, no black. The speaker grid is also different with an imitation bass boost that does nothing and the music desk is also different.
Most importantly, inside of these are EXACTLY the same boards, with the extent that the PSR85 boards are labeled PSR500, so they have the same everything inside.