I'm wondering if there could have been a back door to the described situation.
Remembering this was before micro-chips were available, I'm wondering if a short-circuit across several circuits might have been interpreted as a positive response to several otherwise separate interrogatives. And that the short circuit could have reported three positives from three different places.
People looking at the "front" of the arming mechanism might have seen 4 different switches (or analogs) in 4 different places, whereas behind the panel doors there could ultimately been one circuit which totaled all the other responses. That circuit when shorted could have interpreted 3 "OK's" and just left the 4th to be tripped. After all, something had to add up all the inputs, presumably in one place.