Bat your eyelashes and remind yourself that you are *not* looking at the circuitry for two TH-621 radios in the above photos. Sure enough, the radio on the left is the same ivory unit you've seen on the previous pages. But the photo on the right is of a grey Standard SR-F22. The two circuit boards are identical and are made by Standard.
That's right, Hitachi didn't actually build the TH-621. Rather, it was built by Standard as an OEM product for Hitachi. My suspicion is that Hitachi wasn't able or willing to invest the time and effort to create their own shirt pocket set, yet were eager to put such a product on the market. Standard had already released the SR-F22, was geared up and willing to build OEM products for other makers and trading companies, and they obviously got the contract for this model. I figure Hitachi made a deal to have the radio made in exchange for guaranteed supply of transistors, (the devices themselves), on top of whatever financial arrangements were made. Without sharing my suspicions, I asked a retired Hitachi employee why Hitachi would have made such an arrangement. He just chuckled as though it was common practice within the company and repeated in his own words my theory that Hitachi probably offered to supply transistors in exchange for the product. Transistors were still in short supply in late 1957, so priority access to quality Hitachi devices for their own radios surely was attractive. Here we also get an explanation for the transition from Sony transistors to Hitachi devices at Standard in late 1957.
A number of the other hints convince me that Standard built the whole radio and did not simply supply the circuit boards. This ranges from the serial number pattern used on the earlier TH-621 radios, the product labels, the stickers on the back and more. (*Note: I will add illustrations later to make it easier to compare at a glance.)