The Crowfoot wrenches in the photo below are believed to be the earliest design.  They were first listed in Catalog #9, in 1929. 

 

It's easy to imagine the tool maker taking a piece of square stock, stamping the two holes and rounding the edges.

 

Collector Troy D. from Missouri proved that P&C made crowfoot wrenches in 1928, and possibly earlier.  I've posted a scan of the catalog page.  The P&C model is a more modern design than the Plombs.  Perhaps the Plomb wrenches were designed in a hurry to compete with the P&C line.