Halogen bulbs are smaller than certain other types of bulbs, which means they fit in smaller light fixtures. On this light bulb you'll find a G4 socket.
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Osram 64408 Halostar OVEN halogen lamp
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Osram Halostar 5w Oven Appliance Bulb 12v g4 - 4050300308029
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Osram Halostar 5w Oven Appliance Bulb 12v g4 - 4050300308029