Originally posted by Bjarney...

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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.

Now that you mention it, I think it might have been used for an older water pump that needed grounding. The boathouse where this was found is also where the pump has always been located. The cottage was wired in the late 40's. I guess this was some early design by Hubbell for equipment that needed grounding, but was forgotten once grounding became standard in the 60's.

Thanks to everyone who posted!