T-Handle Gang,
I'm guessing that the reason there are so many T handles out there is because the lower priced socket sets of the 30's, 40's, 50's, and 60's did not come with a ratchet. A metal box, 6-8 sockets (in common sizes), an extension or two, and a T-handle. Simple, inexpensive tool sets for the home owner, farmer, car owner, motorcycle owner, handyman, or do-it-yourselfer.
If you check the old tool catalogs, you'll see these simple socket sets in damn near every catalog during this period.
My dad had an even simpler and lower priced socket set; an L-handle with 7 or 8 sockets that stored on the L-handle. This set was hex drive, not square drive. I originally saw this tools set in the trunk of his 1956 Packard. I may still have this tool set out in the garage somewhere. Haven't run across it or thought about it in years.