From Alloy Artifacts:
"7/16-Drive Tools and Sockets
Blackhawk's line of 7/16-drive tools was one of their more distinctive products, and no other company is known to have produced tools in this drive size. According to the Blackhawk sales literature, the enabling factor for these tools was the extra strength of the Hexite steel alloy developed by Blackhawk.
By the mid 1930s Blackhawk had developed an alloy steel named "Hexite", and claimed it to be 70% stronger than the typical chromium-vanadium alloys in use. As if to back up their claims, Blackhawk designed a new line of sockets and drive tools with 7/16-drive, and extended the claim that these were as strong as conventional 1/2-drive tools but only slighty larger than the corresponding 3/8-drive tools. With these tools (or so the marketing claim went), the mechanic could dispense with the trouble and expense of maintaining both 3/8- and 1/2-drive sets and go entirely 7/16-drive.
Blackhawk produced a full range of 7/16-drive sockets to go with the drive tools, including shallow, deep, and universal sockets. Since this line of tools was supposed to replace both 3/8- and 1/2-drive, the socket sizes ranged from 5/16 up to 1-1/4, in order to cover the normal size ranges of both the larger and smaller drive sizes. "
No wonder that NOS ratchet went for so much. They must be rare.
But to me, at least, they're just interesting collectibles, since it seems that there are no metric sockets to fit the ratchets and accessories.