I got the information from the Telecaster Discussion Page (TDPRI).
I said the old ones,
and wasn't referring to the first prototype ones.
When the single-pickup Esquire went into full production with a truss rod and contol switch in January 1951.
also this:
"Like the two-pickup guitar, these Esquires had a routing cavity in the neck pickup position. Thus, with the purchase of a neck pickup and replacement or modification of the scratchplate, players could upgrade their instrument to a guitar identical to the Telecaster in every respect except for the model decal."
[
www.answers.com]