Per David Blasco's blog, there's this snippet in a post about a J1 James Dean owned (briefly):
"I have a copy of original document that verifies the VIN as J2/7484, a 1951 Royal Enfield J2, 449cc. Purchased used on Nov. 30, 1953 from Marvin Carter of Carter Motors, Fairmount, Ind. for $395. Signed by James Dean."
So apparently as early as '51 they were for sale in Indiana. I suspect that this was due to arrangement Ralph Rogers of Indian had made with Brockhouse (the company in charge of importing REs into the US). If my memory serves me, this arrangement was made in between '49 and '51, which means they were likely imported into MA, then distributed to the fortunate (though, likely, unwilling) Indian dealers across the country. Indian's dealer network was shrinking at this time, but still sizable enough to assume there would be a dealer in nearly every state.
Anything pre '49 would have had to have been a private import, I suspect.
Just my hypothesizing...
Very interesting, my Fathers RE J1 is 1946, I could assume it's been here in the states since then? I couldn't see why anyone would import it but who knows!
If it has been here and there is proof, I suppose that will push 1951 back to 1946 or 1947