It isn't nice to spread mis-information. Esterbrook never used the term "rootbeer". While there were two browns (Brown #1 and #2), they were known as "Brown" (the darker of the two, and "Copper" (the lighter). The term "rootbeer" is not an Esterbrook moniker, and serves to confuse the issue.
I think you should know, there are actually two "browns"... the darker is called "rootbeer", the lighter is called "copper". Of course, the copper is (as far as I've seen) much more common to find than the rootbeer.
Actually, for the double jeweled Js, Esterbrook never used the term "copper" either. They just used the two browns. Earlier pens (non double jeweled Js, pre-1948) had the name of "copper", but after that it was not used. That's according to Richard Binder's "Esterbrook J family" page (about 2/3 of the way down).
But that is really immaterial. If the OP is going to be looking on ebay or other places like that, he WILL run into those two names. It'll be root beer and copper in the descriptions, not usually brown 1 and brown 2.
While I do appreciate the education in regards to the official details, Harlequin is right. 'Root Beer' and 'Copper' are the two terms I keep consistently coming across. Having found several of each in a restored or NOS state, I now have to decide between them.