Zenith build just a few rectangular movements. The most ones are very small and made for female watches. This form enabled a quite big balance depending on the hole movement size. The fashion especially for woman was to wear very very small watches in the 1930 ties. For the popular rectangulär watches those days you find the 3 3/4 F caliber in male watches. The aera to build not only round wrist watches began in the end of the 1920. At first Zenith build a tonneau form with the round 8, 8 3/4 or 10 ''' NSI caliber inside. Not sure when the 8 3/4 came in the market but it must be around 1930. The rectangular watches were sold in the early 1930 ties as well as round and tonneau ones.
Here are my examples and I guess they differ several years in age but I know Hartmuth and Sempervivens will help
both movements are within quite simular but not identcal chromed cases with a steel back. What is striking is the different thickness and shape of the watch glasses, which I consider to be original.
a view on the case numbers let me think that here are some years between both watches while the one with the thick glas is older:
now a view on the movements. The nickel one is just come in and I hope it will soon shine like the cleaned gilded movement. The "dirt" is just old dryed oil and no rust or corroded:
finally the two dials without glass. Photographs are merciless. In reality these are beautiful well preserved watches and they are splendid to see. My luck is that these days small rectangular watches are not in demand.