An archaic and obsolete connector previously used in the theatre, often in floor pockets. And always on Piano Boards. Having no earth ground, and exposed copper contacts, is extremely unsafe, and must be avoided. The 50-amp plug fit into a receptacle approx. 1" x 2". Two 30-amp plugs measuring ~ 1/2" x 2" could fit side-by-side in the same receptacle. A "modern" half stage plug by Mole Richardson, still available for purchase, and still in use on some Hollywood sound stages. A "Half plug," Kliegl model number 301 1/2, recently removed from service: from http://www.derekleffew.com/derek'sblog . A very rare animal, on top, a three-wire (hot, hot, neutral) stage plug, AKA "gounded stage plug", "triangular stage plug": On bottom is a "standard" two wire (no ground) 50A, 120V stage plug.