I found two fat little Round "O" Plombs at the Swap Meet last weekend.  Tools are stamped:
Tool 1
(Front) PLOMB TOOL CO LA CALIF (CALIF is only partially readable)
(back) 7/16 S.A.E. 3/8 (3/8 is inverted)

Tool 2
(Front) PLOMB TOOL CO LA CALIF
(back) 7/16 S.A.E 3/8

  AA shows one wrench of this type, but not sure of the date.  I think that the round "O" and the ID with LA CALIF makes them pretty early, 20's or 30's.  Am I good on the date???

  I had never seen any this thick.  They are around 7/16" thick.

Jim

 They are well fed. Never seen that configuration. nice find.

I like that bottom obstruction wrench. Nice short and beefy design!

Textile patterns, I didn't know plomb made those, very neat : )

Early date range is probably right, does the stamping look like it was done by hand?

The stamping on the front is nice and straight and uniform.  Not by hand.  On the back, where they stamped the size, it is quite possibly by hand.
Jim

 Alloy shows similar wrench at start of open end wrenchs. Lists thickness at .44 which seems to be same as yours.