The speaker is definitely external and there is a receptacle for it on the bottom of the unit.
You can see it in this photo:
It's a three hole receptacle and looks like this (bottom view of radio with front cover removed)
The receptacle is the circular brown cluster of three holes on the bottom left of the unit in the picture. The speaker was a standard, nine x six inch oval, 4 ohm unit with two leads as most are. You can hook it up without that 3 pin connector by soldering some aluminum nails or other such pins to the ends of the two speaker wires. You can also remove the back of the radio and solder a couple of wires to the connections on the inside of the speaker receptacle, run them out one of the air holes and then connect them to a speaker with spade clips or solder.
The bigger challenge you'll have is finding the mounting hardware for the speaker itself. There is a bracket that mounts the speaker behind the grill in the dash (without drilling holes in the dash!) using two holes on the bottom ledge (horizontal surface) of the dash. It's unfortunate as I had the one out of my '53 J (they all used the same one) just this week and I neglected to take photos as I often do. I do know a fellow not far from me who borrowed mine and fashioned one out of sheet metal. I'll send him an e-mail and ask how he did it. He no longer owns the car unfortunately.
AZ_HJ scanned the instalation instructions, which include illustrations so I'll post them. I wouldn't want AZ to conclude that one person not using his info might mean that no one does so here it is:
The club handbook has information regarding Henry J radios and it explains there were actually two types. I don't know which type you have. This diagram shows the earlier type (KF Accessory 107219). Either way the part you need, labeled "speaker support" in the diagram, is the same for both types. AZ may also have a photo of the speaker support/mount, if so AZ can you post it?
