Curious sterling silver pocket watch

All looks completely original in my opinion.

It's an Omega Art Deco Lépine in a silver case (Swiss Bear hallmark and 900 stamp). It has a silvered dial with "Serpentine" hands and Arabic numerals in an Art Deco style.
You are correct about the date, a couple of years either side of 1920 maybe.

Caliber is most likely a 35.5L.

In Switzerland, silver of .900 fineness is mostly referred to as “coin” silver. Not sterling.


0.900 silver

Swiss 900 silver
Sometimes a standard mark of fineness of 900‰ or 0.900 is seen. This was never introduced into Swiss law as a standard, but 0.900 fineness silver was a popular grade in a number of European and other countries because it was the standard of coin silver. Watch cases that were to exported to a country where 0.900 was the standard were stamped by the manufacturer “0.900” as shown in the picture here, and submitted for assay and hallmarking as normal.