Original. Nice.
An ex-military issued watch used by the British services (GSTP = General Service Trade Pattern or General Service Temporary Pattern).
It has an inexpensive but very reliable caliber 38.5 L 15p (L = Lèpine, the style of the watch, 15p = 15 jewels or rubies). The 38.5 was made from 1932 to about 1966.
The movement serial number dates your watch to the mid 1940s.
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