I have seen around, and actually bought one, Bravington's GSTP carrying on the back their mark.
I have not seen GSTP with other makers' marks on the back: I guess there is a reason for it, but I would like to know your knowledgeable opinios.
Thanks a lot

Bravington's was not a watch manufacturer - they were a British jewelry company that sold military surplus such as watches after the war.

Hi, Jim.
Sure, Bravington was (they went in receivership in 2007) a London based jewelier, with a long history, more than a hundred years, and I think that they sold to British soldiers too not just surplus. Incidentally this watch doesn't have the "S" sold-out-of-service mark, so I wonder if you could sell it officially, putting even your mark on, as a surplus. The movement is a Unitas 235.

Good question and I wish I knew the answer.

Do you know the meaning of the XX on the back of this one. I see it fairly often on GSTPs. Is that some kind of sold out of service marking or is it something else?