These caps are used in Halogen Spotlights. They also have two prongs, but are fitted by a "push and twist" action. These are often used in ceiling fittings that are fitted flush to the ceiling and are housed in a circular case. To change these bulbs, the circular case needs to be unscrewed from the housing which is unseen and sunk into the ceiling. The bulb is usually held by clips, which can be undone, before removing the bulb with a gentle twist and pull.