Transistors: (Hitachi) HJ-23D x 1, HJ-22D x 2, HJ-15 x1, HJ-17D x 2, [1N34A]
Output: 120mW (distortion free), 170mW (maximum)
Power source: 6V : UM-2 (C-cells) x 4
The large speaker and ample output produce excellent sound. Hitachi's soft plastic "leak-proof battery cases" were pretty slick. They have a cap at one end and the holes at each end are just large enough to allow the batteries to make contact. We find these battery cases in a couple of other early Hitachi radios as well, including the TH-668 and WH-667. You can make out the Hitachi mark on the electrolytic capacitors, but a peek underneath reveals that they are OEM parts made by Nippon Chemi-con.
Note that the oval transistors pictured above were only used in the TH-667 for the first few months of production. Units produced after August or September 1958 contain cylindrical transistors of the next generation.