I'm just wondering out loud here, but are the tangs actually engraved? I thought they were stamped. If they were stamped, you would probably have one line for American import razors to meet the tariff requirements that could simply be left off non-exported razors... I tend to rely on the tail length, grind, blade style, and sometimes the scales if they appear original, as well as the tang stamp. Of course, I'm no expert.
You have it right, they are simply another clue to be weighed against all the rest. You have to understand that it worked both ways too... Some of the older English Marques were rather proud of their "Sheffield" steel and had the "Made in Sheffield" way before 1891 As long as you don't think it is the "Only" definitive dating clue you are fine, just look at the Razor as a whole, instead of just that stamp This is a good read on it http://www.coxsackie.com/reference/cooaadt.htm