12V Snap On Power Cords, or Female Plugs for a Snap On Optical Toe Gauge WA110A
Those are a type of "Jones" (Cinch-Jones) plug and may be obsolete. They originated in the '30s and '40's. Most of them you likely have seen are rectangular, and have the characteristic black bakelite connector with an aluminum black crackle cover.
EDIT Mouser still shows some rectangular ones, I did not look too hard