This Taubert & Fils case, with the Borgel FB-key trademark which they used, has an unusual screw in back. I have never seen one like it before, it appears to be a transitional stage between the old Borgel one piece screw in case, which they were still manufacturing, and the decagonal (10 sided) back case which the Tauberts patented in 1931.
The milling is intended to give you a grip to unscrew the back, but when it is screwed fully home the back is pretty well flush with the case so it's difficult to get a grip. There are some tool marks which don't show up too well in the picture but look like someone has used a wrench to open it at some time.
Although the inscription is dated 1937, the case back carries London import hallmarks and the date letter n of 1928/29, nearly 10 years earlier. Did it really sit on a jeweller's shelf for nearly 10 years, or was Sgt. Fisher given a second hand watch? The DCLI I think stands for the Duke of Cornwall's Light Infantry.
The sponsor's mark is DBs showing that it was imported by Dimier Brothers. The Swiss cross and the "BREVET" imply that it is a patented design, but I have not been able to find a specific patent for this case under either Taubert or Dimier.
The middle part of the case has some details which are similar to those illustrated in this picture from the Taubert's patent CH 130942 of 1928. The back of the case (2) has an external screw thread like the patent illustration, and the case middle has a channel to take a gasket (9), although there is no trace of the gasket.
The stem tube is also internally flanged at each end as if to take a cork seal like the Tauberts used in their decagonal back cases, shown as item 13 in the figure, although again there is no trace of a cork seal.
The way the movement is held is unusual too. The movement sits in a carrier ring, but the carrier ring is not threaded, it just sits in the case and is held in place by the case back, very much like the one we were discussing a few days ago from a decagonal case. There is a slot in the carrier ring of this watch that engages with a key inside the middle part of the case, which wasn't used on the later decagonal back cases.
The watch has been robbed for parts, there is no dial, hands or stem. It's a Fontainemelon movement.
I would love to see a complete one of these, if you have one or know anything about these cases, please get in touch or post details.
Regards - David