This porcelain lamp socket was originally used in a Packard Pla-Mor jukebox.
Two of these mount on each side to illuminate the large corner plastics.
This is a Medium Base lamp holder used with common 7.5, 15 and 25 watt lamps found in vintage juke boxes.
Two of my photos show the upper cabinet interior of a Packard Plamor for reference purposes.

The two mounting holes in the metal bracket are spaced 1 inch apart center to center - the overall length of the metal bracket is 3.25 inches.
The distance from the center of the lamp socket to the edge of the mounting flange is 2.25 inches.

It has been washed and is very clean.