FD,

"I suppose it's also possible the battery manufacturers were giving
the low end transistor radio manufacturers free batteries in order to
later sell short life, high profit replacement batteries."

Sounds like the early Smoke Detectors with thier Odd Voltage
and Big Odd Size Batterys that some times cost more then
the Smoke Detector did when new.

"The Japanese companies prefered the now familiar rectangular
battery which had a stack of 6 rectangular cells,"

This is what I remember from the 50s/60s a stack of Six Waffers
about 3/16" Thick {Rectanglar Cells} inside the 9 Volt Batteries
of that time.

~ RHF