it is sort of odd that they used the fancy, but expensive,
7360 balanced modulator tube.

That was an extremely common choice for not only for not only
transmitter balanced modulators but also for receiver mixers and
product detectors in the 50's and 60's. I think it must've been the
case that one early ham article used it and everyone else copied it.
Certainly it was not a common junkbox part...

Tim.