These weren't three aspect signals per se. The bottom aspect was an illuminating 'S' for when slam door trains (Dreadnoughts and T Stock) were ready to depart. The tip of the guard's flat touched wires and illuminated an 'S'.
Thanks I had forgot all about the 'S' signs, I knew that the Met had 3 aspect signals futher out did these also have the 'S' signs the other thing is thinking about it I thought that they were a separate aspect.
This is only a recollection from many years ago when the 'T' stock was running, early sixties at latest, so getting on for sixty years ago!!! I then lived at Pinner and the signals were three aspect R/Y/G at the platform end. I do not recall any sub aspect showing 'S' or anything else for that matter. The guard would touch the brass end of his flag stick against two wires suspended above the platform, a bell would ring at the other end of the platform and the train would start. The guard was usually standing fully on the platform when he did this, he would step onto the train as it began to pull away. I often thought that any slip on the part of the guard would leave him stranded on the platform as the train trundled away!! Never saw that happen though!
Yes believe it was either a bell or an illuminated ‘s’ or both which was given.
Of course the whole lot ceased to be used from the early 60s with the demise of the former MET stock. I understand the bell would ring at both ends until some bright spark realised this could cause the opposite train to depart!!
I’m sure I’ve seen independent aspects (like trip cock testers or fixed red lights) in the odd location with an ‘s’ on. A similar system used to exist at Finchley Road to singal the adjacent train.