James Farrar wrote:

At Monument westbound, the Central line can be signed either one of
two
ways using such a sign.

Yes, Monument/Bank is an obvious example. Also at Tower Hill, I believe
(for directing people to extra exits in the peak), and Waterloo as
mentioned. London Bridge *tube* has the illuminated ones for signing
alternative routes to the Northern and Jubilee using both escalators,
whilst the NR station does indeed have rotating flaps to indicate the
peak tidal flow system.

What about the Waterloo & City line? It's been ages since I used it.