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... there is one thing of signifigance, and thats the fact of the ground pin being longer than the current carrying blades...

Well, I don't have any crowfoot plugs handy (I've rarely ever seen them), but several such are illustrated in the 1960 WESCO catalog. They all appear to have longer ground pins.

What represents an improvement in the 5-15 (apart from backward-compatibility) is that the u-shaped ground pin is too wide to be inadvertently inserted in a live slot.

Although T-slots and older parallel devices have a 250V rating, I doubt they were ever used much on circuits over 120V, since appliances with these plugs were, and are, so common.

Any other plug could be used within its maximum rating. Crowfoots were used for 240V circuits, and for 120V grounding circuits.

I need to get my scanner hooked up, so I can post pages 198-199 of House Wiring by Thomas W. Poppe (1930). Two methods of grounding a washing machine through the cord are illustrated. One involves a crowfoot, and is essentially the same as the modern method. Let's just say the other is scary! [Linked Image]