Here is what I have been able to piece together of the J1 system bus. AD0-7 and A8-A15 are the multiplexed address and data pins. You can address 64k max directly through the system bus. RD, WR, ALE, IO/M, NMI, IRQ are z80 style pins but there are a few weird unknowns.

The PA0..PA6 are the peripheral select pins after some IO decoding and share interrupts between boards. I only decoded the two.

These pins are from the 4951C drawings. Seeing as the same floppy board is in both machine I would expect the pinout to be the same. Hopefully?

BGRANT

BREQFLP (Bus Request Floppy I assume)

RAMEN / X6

RE

M4

M16

Pin 31

Pin 20