I don't know how meaningful this is, but there is a rifle on GB advertised as a 99 G in .250-3000. (No serial number is given or seen in the photos, but the description says the rifle is from 1918-1919 which would make it a second variation Model .250-3000.)
Accompanying the rifle is a .410 barrel with the single line barrel address. No way to know how long the .410 barrel has been with the rifle, but the fact that it is with a rifle from roughly the same time as the introduction of the .410 barrels tends to support the theory that the one line barrel address .410 barrels were the earliest ones.
