This is a nice watch. The British Government contracted to buy several thousand American and Swiss pocket watches for the military at the start of WW2 as they had poor stocks. They didn't actually specify movements as the G.S.T.P. (General Services Trade Pattern) standard specified so many jewels, such-and-such a face, etc. So the watches purchased and issued might have Waltham or Elgin movements from the US, or Cyma, Leonidas, Omega, Rolex and other movements from Switzerland. The pheon (broad arrow) marks it as British Government property, and the G.S.T.P. mark and the serial number mark it as probably Army issue. I forget the meaning of the individual serial number codes at the moment.
Cyma/Tavannes made excellent movements and still do. Worth getting the back off and giving us a pic, I reckon!