Early Power Sockets

[snapmom]

There is not to much known about early power sockets. They show up in the 37 industrial cat, but they were made starting in the 20s. So there is probably a 20s industrial cat out there some where. There are some oddities about these early ones, but I do not have enough examples to compare, they are hard to find.
In the pic below, left to right, P416 1/2 square opening, 1/2 dr. 1927 date code. Second, P340 1-1/16 opening, six pt. 1/2 dr.1929 date code. 1929. Third P400 1-1/4 opening. six pt. 5/8 drive 1927 date code.
Second pic. P-823 2-9/16 opening. one inch dr. six pt. no date code, (I would date it 30s)
If you have some of these early power socks, post some pics.