However, the output port is decoded on the decoder board using bits 4..1 of the instruction register. They're exposed on pins on the processor boards edge connector.
The instruction decoding for the INPUT command looks to be partial enough that it is issued irrespective of the port address. If so another board could be decoding I4..I1 to give an input port address.
(But the pinouts for every board are unique to that board. It's possible no other board has access to them).