I think the open comb and damaskeene versions were the oldest, anything with a plain knurled round handle is the pre 1920s versions.
All the ones with a hexagonal handle (and all versions of knurling and patterns of that), were made from around the 1919 or later. 1919 was when "Gem, Star and Ever-Ready are joined under one banner, the American Safety Razor Corporation" - Taken from ASR personnas website where they mention the history of the brand.
Personna | The History of Personna American Safety Razor Company
I've got a razor and strop set from GEM that's british made, way different handle, and heavier than any 1912 razor I've seen, and Dave said that was probably from the 1950s! And I'm sure they made a lot of 1912s for WW2 in the 40s, that was their main razor until the plastic handled ones.
Edit: My GEM damaskeene is labeled 'Gem Cutlery Co, New York USA'. Later hex or patterned handled 1912s are just labeled 'GEM Brooklyn New York made in USA'
Also my Pat 1900 lather catcher, which Dave said was probably a 1911 manufacture date, also have the same thing as the damaskeene razor, 'Gem Cutlery Co, New York USA'. So damaskeene models are probably around 1912 date of manufacture, maybe 1914?
'Damaskeene' is a blade they were selling, the razor has "only use damaskeene blades' stamped on it. Probably in competition to ever-readys blades and pat 1914 razor (as stated, ever-ready and Gem were separate companies until 1919)
Every-ready was started by someone who left the Gem company