Antique Military Issue WW1 Pocket Watch

A nice, working (though a clean and lubricate would, no doubt, help enormously)Military issue pocket watch in a screw back and front case. It has the Kings Broad Arrow on the back together with the identifying letters GS-TP and the letter S beneath the arrow. The military number is 042512 and it is inscribed "Bravingtons", who were a top class retailer at the time. The movement is also of very good quality, being 15 jewel and fitted with a Breguet overcoil hairspring. The only problems are a seriously yellowed crystal (easily replaced), a little corrosion on the case (which looks like chrome on base metal) and a ding on the back seal where some vandal has attempted to lever off the back, not realizing it was a screw fitting (see photo). All-in-all, a nice addition to anyone's collection of WW1 memorabilia.
I am told by someone who knows more than me that the letters GSTP stands for General Service Time Piece and was a designation only used during the Second World War. I would not think to argue, I do know enough about militaria, but I was under the impression that pocket watches were only issued during the First World War.