that's it. Chev 6 oil pan screw socket.
made by a number of manufacturers.
Thorsen 10211 - shown on page 23 of the 1938 Thorsen catalog
not sure if Indestro made one... I'll bet Duro did, though...
How did this hold the screw? Was the screw captive or was it simply a collar to keep the screw and socket centered and relied on gravity as the screw was offered up to the bottom of the oil pan? How about horizontal use on the tappet cover?
Joe