Doug:

It was the wide use of these tubes (along with others like the 4-250 and
4-1000) in the TV and FM broadcasting industry that made them so plentiful
and allowed "used pulls" to be so readily available to hams. Amp designs
using them have been around for decades. I just looked in the 1965 ARRL
handbook and they have a 4-250 amp for 6 meters. A little homework should
turn up many more.

Good luck,

Roger K6XQ