This arrived today. Quite big at 34mm:
The movement is Swiss with 15 jewels:
And is marked B & Co. with 3 little stars:
which makes it a Baume Freres watch (Baume & Mercier). Which I didn't know when I bid on it.
The movement is also marked 13.34.
The case is solid silver with the London import mark for 1923. And the AB mark of Arthur Baume - a casemaker from the same family: "Arthur Baume, son of one of the founders of the Maison Baume and director of the Baume & Co subsidiary company in London."
The crystal is thick, somewhat chipped, glass. The back is screw on.
And it has hinged lugs:
and it needs a new mainspring.
Edit: It now has a new mainspring. The serial number, on the dial side, is 4042968 which is the Longines sn for a movement of c.1922 - just before the case (1923).