Does anybody know anything about these old jewels?

I was asked to look at one today as it wouldn't carry an arc. I'm a pretty good electrician but know little about welders or how they deviate from being a simple DC generator.

Is there any resource with perhaps a wiring diagram or manuals? I'm thinking there's a big selenium stack looking thing on top with engine vacuum going to it. I suspect that's our regulator and our problem. Maybe controls excitation somehow....

Anybody know of any resources?

Thanx, Steve in Las Vegas