MES/E10 Bulb Holders

Although these take a screw bulb the holder is not fully threaded but just has these two bumps on each side:

Visible as indentations on the outside. The claws make it a bit of a fiddle to screw the bulb in as you are pushing the centre contact of the holder back against spring-pressure as you screw the bulb in - this one is only partially fitted. If you hold the holder with one hand and pull back the wire and hence the centre contact with the other, then grip both with one hand, the bulb screws in more easily: