MattE, that's quite a find. The “crowfoot”/parallel-blade all-in-one combination is curious, and as NEMA wiring devices go these days, looks like a combination of NEMA 1-15R and 10-20R, but these days, the 10-20R configuration is for ‘non-grounded’ 3-pole 3-wire 125/250V 20A service. Maybe a 1940s mutation for earlier 120V-grounded portable equipment? I think NEMA 5-15R and other “U-ground” devices were a mid-/late-1950s iteration.

The “small crowfoot” (NEMA 10-20R) illustration is…
[Linked Image from 6l6.net]
At that time, differentiation between a current-carrying neutral and an equipment-ground conductor did not exist in wiring devices. Out west, 10-20R was the predecesor to 3-pin NEMA 6-15R/6-20R for grounded 240V, cord-connected window air conditioners starting in the mid 1950s. 240V {up to 3.8kW} portable electric heaters [probably as early as 1930-40s] typcailly used a NEMA 2-20R 2-pin 2-wire, NO-equipment-ground configuration, and occasionally 2-30R {up to 5.7kW.}




[This message has been edited by Bjarney (edited 07-11-2004).]