On the 155, the trigger/gate board connects to both trigger/sequencer boards which in turn connects to the jumpers on the SOLDER side of the 154 main board. That's basically one long ribbon with four connectors.

Both modules are powered by one power ribbon which is on the 154, bottom left of main board, below MC14551BCP chip.

JP4 on the 154 connects to ST2 on the 155-4 control PCB.

JP3A and JP3B are both on the 154, near the top, and connect to one another

Nick