Gareth's was W7 now W10 Downstairs Computer explained :
That's interesting, for assuming that it predates the incursion of
electronics by several years, your description of differing operation
front and back suggests to me some form of DC operation entailing
a permanent magnet.

So far as I remember, they use two separate solenoid coils and two
terminals. One coil fires it one way, the other the opposite way.