The published co-ords are for access to the subway, there is no cache there.
Back in 18AB the subway was constructed to take the commuters from the Crystal Palace High level railway to The Great Exhibition.
Unfortunately The Crystal Palace was destroyed by fire in19CD and then the High Level Railway was closed to passengers in19EF, it was later demolished in 1961 leaving the subway isolated.
The Friends of Crystal Palace occasionally grant access to the subway and queues by the gate can be seen stretching down the parade.
Cache can be found at N 51 25.CC(A-B) W 000 04.D(A-D)(D-F)
(Checksum for all digits = 32)