I just acquired an R-4b (serial # 9737C) and it has a 1/4 phone jack between
the RF gain knob and AVC switch. Don't see this jack in any info that I can
locate.The receiver has been stored un-used since being re-caped and etc by
drake just before drake quit taking in and rebuilding things, probably 8 or
9 years ago. Would drake have added something? The relative who owned it was
not inclined to modify equipment. RM~

PS, No I haven't opened it up or fired it up yet, just un-boxed and studying
things.