The trigger signals are shaped on the smaller KLM448 on the left. If i recall correctly the digital board sends very thin positive pulses to theKLM448 board via the wide connector, which are then converted into the correctly shaped negative going pulses by the circuitry on the trigger board, and sent onto the synth board.
For each sound theres a small cap just after the input to the board. I think you can inject inverted trigger pulses on the circuit side of each of these caps to trigger the sounds. Just shorting these points to ground with a wire should trigger the sounds. I wired up a load of separate transistor inverters to get the negative pulse, which you can see on the small board to the left of the green sequencer board on the bottom image.
I do have a vague recollection of having some trouble with the external trigger pulses only working when the digital board was disconnected, or that could have been the other way around. I've blanked the whole thing from my mind after the amount of stress it caused me
Give it a go and see what happens.