These plugs (officially BS546) comes in 2, 5 and 15 Amp versions and are still used in stage lighting installations, large country houses and hotels. They have their own dedicated circuits which are fused.
Normal 13 Amp sockets are wired in a Ring Main. Circuits containing BS546 sockets are wired "radially" back to the fuse board and are usually fused at 5 Amp or 15 Amp.
How are these to be PAT Tested?
The Portable Appliance Test is just the same as for other appliances with the 13 Amp (BS1363) fused mains plug. Identify the appliance as Class 1 or 2 and carry out the appropriate tests using a suitable adaptor.
One will not be able to check the fuse, as the plug does not have one. However it is good practice to ensure that the wiring has been inspected recently (in the last 5 years) by a competent electrician.
The safety of an appliance with a round pin (BS546) plug is provided exactly as other appliances. The main difference is the absence of the fuse in the plug.