What Kind Of Outlet Is This???

Did a home inspection yesterday and found this anyone familiar what kind of outlet it is and any info

Its called a twist lock plug . Made so the plug must be twisted to lock in and twist the opposite direction to remove . .
No idea what voltage receptacle is .

https://www.google.ca/webhp#q=twist+lock+plugs

Anyone know what this is for?

David, that looks like an old twenty amp outlet (no ground).

Wow google is awesome

http://www.electrical-contractor.net/forums/ubbthreads.php/topics/197738/Some_vintage_receptacles.html

http://www.greenbuildingcentral.us/home-wiring/i-i.html