Thanks Silver965, they are almost identical looking expect the 99 has a silver plate for a tape guide. The French Service Manual for the 77 is on Electrotanya but I don't see the unit part number and I can't find a 99 Service manual.
SONY CFS-88L Service Manual download, schematics, eeprom, repair info for electronics experts (elektrotanya.com) see also cfs 88
It would make sense for them to be the same deck.....i doubt they would re design the deck just for a model up....fixed m90's many times using parts from different decks
Another example is Aiwa AD-Wx110.. Aiwa cs-600... Aiwa cs-770... Aiwa cs-880... all the same mechanics.. I think there are still others but I have not deepened the research