This was one of the training masts at Ganges, situated on the foreshore at the bottom of the Long Covered Way. The mast was used to train signalmen (buntings) in the use of flags. The platform was used to train boys in lead swinging.

See also https://pbase.com/hms_ganges_museum/image/169601776 this one looks to be on the upper sports field.

For a view of the platform see this one https://pbase.com/hms_ganges_museum/image/169243225