This house had sockets that would accept NMEA 5-15, 5-20, 6-15 or 6-20. (I forgot the numbers until you mentioned them). It wouldn't be allowed now, but my guess is the house was built in the 1950s.
The NEMA 6 series is wider than the 5 series and the 6-15P probably won't fit in those old outlets, but the NEMA 2-15P would. . . if you could find one.