Why only one speaker in the rear?.

Its a 8 ohm radio. 2 8 ohm speakers is a 4 ohm load. That's what you have with one in front and one in rear. To briefly examine the word "load"... Anything with a load rating including chain hoists and other material handling equipment will handle some degree more than its advertised rating. That's when new. Don't attempt to lift 1.5 tons with a 1 ton hoist if its a 100 years old. LOL Back to the radio...
There output transistors would handle that 4 ohm load, a lot better than the modern electronics atleast. You could kill the old factory radios if cranked wide open for extended periods. They were more often killed with additional speakers/load.