If you are talking about the bracket that holds the socket...that's actually for either mogul socket or a medium...looks like this one got medium base sockets!
I see what you mean. There's a provision for a house and/or street side socket. The house side one might just be a cut-out for a socket. Sometimes certain distributions are street side and others can be obtained better with a house side socket.
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Still trying to find out who makes those!