Yamaha CS80

Hi all, 
after completing (9 months work...) the overhauling of the CS80, that  is amazingly playing now, I have opened a friends' CS60, that doesn't work (as usual... :-)
I am surprised for finding in all of the the M-voice boards two ICs, named m51623p  and m51628p, instead of the IC8 and IC9  IG00153 and IG00158... 
Is this normal?   BTW, no sign of desoldering. All of the traces are immaculate.
The machine is pristine, and only two voice seem to play some sounds...
If they are correct (a newer  or elder series?) is  any modification needed?
Thanks 

Fernando