GS and Tropical
Hamilton G.S. ["General Service"] watches were specifically made for the UK's General Services. They were actually issued to Government personnel, that were not Military. These watches were used by a number of Government agencies overseas, Foreign and Commonwealth Office aid groups helping with education and health stuff like that from the mid 60's to the late 70's. They also acted as a cover for a number of former military men who helped nations organise their own security and law enforcement on behalf of HM Government.
The G.S. Tropical seems to have been a specially sealed series of watches for use in tropical climates. They are marked "Tropical" on the stainless steel case back, These are rarer than the standard G.S. watches. The case and movement appear to be the same for both watches. The Tropical does not have the broadarrow found on military watches of Great Britain.