 
Socket lampholder with with switch (Bryant BECO F-8428). These are also called cleat socket lampholders with front wire screw terminals. Note that these have a different mounting screw width than the standard lampholder sockets without the switch. The mounting screws are 1 3/8" center to center. The outside dimensions of the base are approximately 1 3/4". The switch handle sticks out about 1/2" from the base and is about 1/2" x 1/4"
These fit into a fixture that looks like this.
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