hi guys Here is something I picked up recently A Clinton manual wind in a sq clamshell case ! 28 mm x 29 mm and take a 20 mm strap ! Cool or what ? Good hunting Bill
Here's my Brev. 189190 waterproof clamshell Tissot, circa 1958... ...and a full length write up about these cases by David Boettcher, here: http://www.vintagewatchstraps.com/waterproof.php
David's excellent article makes some interesting comments on clamshells and square watch cases. He writes: "A Swiss "brevet", abbreviation "Brev.", or patent No. 189190 for a waterproof case design with a distinctive "four-screw" method of retaining the case back and applying compression to a gasket between the back and the crystal was granted to Schmitz Freres in 1937, with a priority date of 11 May 1936. Reference to this patent is seen in watch cases with the legend "Brev. 189190", usually accompanied with the Swiss Federal cross sign . The basic idea of the patent was to use four screws to clamp the back to the body of the case. To assemble a watch the case back and crystal were dropped into a recess in the case body, and the four screws were tightened down to hold the case back in place, and also to put pressure onto the joint between the case back and the crystal, making a waterproof seal. The figure from the patent reproduced here shows how this was achieved. Figure 2 is a cross section, the case back is item "a" and the crystal is item "b". There is no gasket shown between the case back and the crystal but I am sure that one would have been necessary to achieve a waterproof seal. Cases of this patent waterproof design were used by a number of other watch manufacturers, and the patent number 189190 appears in watch case backs. These are often referred to by collectors as clamshell cases after the Gallet original... The design is not as good in principle as a screw back, because the clamping force is applied to four separate points and the case back must be made sufficiently strong that adequate clamping force is applied to the gasket mid way between the clamping points. A screw back is a better solution for a round case because the pressure is applied evenly all the way around the case back joint by the screw thread. The 189190 case design does however lend itself well to "forme" or shaped cases, which are difficult to make waterproof." Source: Information/images from VintageWatchstraps.com © David Boettcher