[Chas]

The "Variac" is fine or a "dim bulb" (100) watts), works too as it is evident that some high temps once existed. Will need a 1/2 hour at full voltage to access if there is low level short, Be SURE that pilot lamp sockets are not shorted. Transformer should be just slightly warm, no crackling, sizzling sounds.

Measure the distance between the rivets to find a socket that has matching holes. Use fiber washers under the nuts so not to strain porcelain. Bugger the ends of the threads with diagonals in event the hardware ever loosens the nut won't get by the spoiled threads...

I have some used porcelain 4-pin if you can't find the spacing. Or drill new holes. Some, old school sockets had an extra springs to grip the terminals... Transmitter tube sockets...

GL

Chas