Actually, it uses an Ohmite Variable Transformer.
It is a glorified adjustable AC voltage & current monitor.
I built this in a vintage Western Electric box with home-made new panels of laminated Formica/plywood, like the originals. Lots of precision wood-craft here, but you wouldn't know it.
In keeping with the genre, I used cloth wire, vintage parts & hardware, except the the Neutrik Powercon connectors.
She will do 500+ Watts, just the thing for those boatanchor bench tests.
I have quite a few of these boxes to repurpose, they are really different from the usual chassis/panel enclosures.