I have a rather plain Jane Hamilton 916 c. 1937 with a gold hallmark on the inside case which I cannot identify. It looks like the twin serpent logo of the medical profession. Nothing else. What does this designate?

I think it may be a Joseph Fahys gold filled mark.

I agree with Kevin.

Check P. 37 of the 2015 Price Guide or google Fahy's caduceus case mark then click images.

I see it in Shugart now. I looked past it because of the other images right near it. Thanks for your help.

Just to add to the discussion, prior to the federal law requiring gold content to be marked on watch cases, around 1921 or so, makers used their own trademarks to indicate the quality of the case. That included a host of hallmarks and the often found 20, 25, or even permanent marking indicating how long a gold filled case was expected to last. While the Shugart book is very helpful it is by no means a complete list of what I believe are the hundreds of individual trademarks that can be found, and of course fraud was as prevalent then as now. Unless you know for sure make no assumptions about gold content and if not marked always assume it is not solid. I collect Hampden watches and admire the Dueber watch cases as well made. Further I do not believe that John C. Dueber, a case maker by trade, ever fraudulently marked cases to indicate they were solid gold when they were not. However I still see eBay adds saying a Dueber case is solid gold when it is marked 14k extra. The case may be nice but NOT solid gold. Ignorance is expensive and early on I paid for my education. I bet Buster has a list of gold and gold filled marks.