Well, the question provided some fodder for the chewing over last evening over a beer with some pals, one of whom is ex S&T, and another has something of a reputation for knowing about these matters.
We came to no hard & fast consensus, however, there was considerable support for the idea that the block instrument and frame had been disconnected, leading to the thought that some other form of protection would be required.
It occurs to me now that a rule 55 collar on the entry point lever would do nicely, but that doesn’t help answer the det question, indeed, perhaps the reverse. I guess you hinted at this in the original question.
There was a suggestion that the dets were placed to warn the returning train that it had to stop, but I’d assume the driver signed for the route, so he’d know that already, there was presumably a signal, and he had to hand the staff in, so that doesn’t float my boat.
They could be there to warn the signalman, presuming they were placed in his earshot, of a runaway vehicle, or indeed the whole train’s approach, because he’d have no other advice of its impending arrival. Quite like this idea. Does the gradient support it?
Best
Simon