I have a lamp purchased in the UK, I can't work out what bulb goes in it. I have tried a bayonet bulb which definitely doesn't fit. I wonder if there might be a missing piece.
The maker code
Dencon 1415 says it is a normal UK Bayonet Cap (B.C.) lampholder with a removable skirt (often used for holding a shade ring). The section with the bayonet slots has broken off. It may have come out along with a bulb when that was removed. Possibly someone unfamiliar with bayonet bulbs damaged it by turning a bulb too far. You need a new lampholder. The brass pins may be dangerous to touch whether the switch is in the 'on' or 'off' position. You should call an electrician or discard the lamp.