Bingo. I found your case in page 152-3 of Goldberger. It is a Cal 13ZN. The example in Goldberger, which is a single pusher version, is manufactured in 1936, so very consistent with what you know about this watch. The case appears to be Swiss rather than American and 18K gold. Dial is very different - one of the cut-out dials popular in the late 1930s.
When you do open it, the casemaker's hallmark should be hammerhead-140.
better and better....
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