old transformer

This is a transformer from an old Welsh 2606k test power supply. I could find no manual or info on how this was supposed to work. I believe it is from the 40's / 50's. (?) I almost called the later company Sargent-Welsh. It came with a rectifier built by Mallory on a late 1930' design. That is as far as I got. I have a compressed photo which hopefully shows where an arm contacts the windings to vary voltage within selected ranges. You can see the grooves in the windings hopefully. Is this an autotransformer? I suppose I could take it apart. Any info appreciated.