Hitachi opted to use socketed transistors on the TH-669. I suppose they were still worried that they may have to swap out failed or under-spec devices. This kind of overkill can also be seen in the first transistor radios of a couple of other Japanese makers of the day, one example being the NEC NT-7. The transistors should be easy enough to spot in this photo, and you can make out the glass 1N34A diode at the bottom right edge of the circuit board, (immediately below the speaker magnet.)
Transistors: (Hitachi) HJ-23 x 1, HJ-22A x 2, HJ-15 x1, HJ-17 x 2; [1N34A]
Output: 60mW (distortion free), 100mW (maximum)
Power source: 6V : UM-2 (C-cells) x 4
Here you can see the underside of the socketed transistor holders on the print side. In addition, Hitachi proudly added the company name in Japanese to the upper edge of the circuit board. Nice touch.