I'd like to reiterate my suggestion of checking the top edge of the chip which is not covered by the heat sink.  The chip label might be high enough that it's not covered by the heat sink.  It's hard to tell how much lap there is from the photo.

The name on the ROM, "proquadra 50" does seem to indicate a 50 MHz 68040 upgrade.   No one ever actually shipped a 68060 upgrade, right?

The Centris 650 was easily upgradable to a Quadra 650 equivalent by moving a few resistors....