Looked at your machine pics again and I believe that unit uses a mag amp/saturable reactor to control the output current. Looking at pic #7 it shows a 3 coil transformer. Two end coils are the high current coils, center coil is connected to weld current adjuster.

Need to check that the DC supply for the magamp is working. Magamp saturates the center leg of the weld current transformer. Vary how much current changes the unit produces. This unit was produced before there were commercial semiconductors (read cheap) that would handle current. I have Miller OM 321 pulled up. It is free download and has schematic. Can't say if manual will apply to your unit, but it will help you tell what you looking at.

Your machine is very similar to my Eutectic machine that I bought (2) from Gov surplus and and made one working unit. Mine has HF on it. We used it to cut lids off of transformers using a Airarc torch. It has reactor control too.

Good Luck

James Geraci