The backplate is a brass round disk with 3 fixings holes that is designed to be mounted onto a piece of wood.

This was the old school way of attaching lampholders onto table and floor standing lamps, and is commonly seen in older style fittings.

However, the older fittings would have had a 1/2″ thread on it, the most common option now is the 10mm.

Below are images and dimensions of the 10mm version.

The 1/2″ have the same base size and thickness, just the thread size is different.