The FPU was optional for the A3630 and the oscillator supplies the FPU clock if the clocking of FPU is set so asyncronus by the jumpers. Most of the A3630 only have a socket for PLCC FPU that only go up to 40 MHz and only the holes for a PGA-Socket. A few actually had the PGA FPU Socket soldered in allready. Maybe only the early ones and it was skipped in later productions runs to cut cost, as those who wanted mor power usually got an A3640 insted of upgradeing the A3630.
