Utility SES replacement lampholder, often used to repair the desk lamps.  This lampholder is a good one to use for repairs as it has screw terminals (not push in ones) and it is a 3 part lampholder as opposed to the clip kind which have the habit of snapping off when you've finally got the skirt in place.  Note this lampholder is double insulated and doesn`t have an earth terminal.  With the Threaded skirt it means you have a very versatile system for mounting a shade or indeed mounting the lampholder in a cavity.  Often used in decorative lighting signs. 

Fits a 30mm shade hole

If in doubt consult an electrician.