Pin 5 looks OK, then.
It may be that the SN7473, IC4, is defective somehow.
Another question, though:
With IC4 removed from the circuit, can you test pin 14 of IC4, on the circuit board?
Pin 14 is controlled by TR2 being on or off.
If you ground the base of TR2 with a jumper wire, then pin 14 of IC4 should switch from low to high.
The TR2 base pin is designated as pin 14, on the edge of the Single Shot Board.
If you don't see pin 14 of IC4 switch, when you ground the base pin, then maybe something is wrong with TR2.
Also, it would be useful to know if relay RL601 is working .... making good contact... as it grounds the base of TR2.