From what I can gather on the net, When Omega bought the Regina name around 1915, the briefly used the name in the USA before it became essentially a "Canada name only". This serial number dates to c.1916, but presented in 1918. Case solid gold which is unusual for a trench watch & is really big, 31mm (very big for the time esp for an octagonal case (another scarcity) & cased in America w/ Elgin 14k gold case (original). Add the enamel dial &you have an extremely scarce if not "rare" (I know people that that word) Omega. I searched a lot for a comparator & could not find one exactly like this. Can you? The owner is is doing research on this for his collection of Trench watches.