Odd Bulgin(?) connector

[ppppenguin]

I went to test an old Murphy monitor a few minutes ago and am stumped by the mains connector. I thought it was the usual large diameter Bulgin, the sort that you have pay silly prices for. So I grabbed my lab Bulgin mains lead and it's not right. The inlet has a lump where I was expecting a keyway. It needs a mating plug of the type shown as #5 here: https://www.plugsocketmuseum.nl/Bulgin1.html

Has anyone seen this type before?

For test purposes I'm afraid it's croc clips. If the monitor goes into service (it's potentially a spare for the caption monitor in BECG's "Southern" truck) then either we've got a suitable connector or we'll change it.