The G5.3 is a ceramic block 2-pin cap with 1.56mm diameter round pins. The distance between the two pin centres, as its name suggests, is 5.3mm.
The G5.3 is found on halogen theatre lamps, studio lamps, and projector lamps.
Miniature Bi-pin, Miniature 2-pin.
Halogen Projector/Stage/Studio Reflector Lamp, Incandescent/Halogen Projector Bulb (capsule).