I have seen older kitchen/bath fans that had the fan motor inside the duct; a short cord & plug went from the motor to a receptacle in the wall of the duct.
Here's the connection: the plugs/receptacles in these fans were of the "y" type that your mystery receptacle would fit (in addition to a "standard" plug).
I believe that what you've found is a replacement receptacle for one of these fans.
Remember- NEMA patterns are not all that old; nor are they mandatory even today. One of my greatest challenges is getting the customer to SHOW ME THE PLUG that is intended to go into the receptacle he want me to install! Then, if the plug is non-NEMA, my next challenge is getting the parts house to put away their NEMA cards!
