That is an older Miller built machine. It is a moving core transformer, and will not have remote amperage capability. You set the amps, and deal with it.
The foot pedal or torch mounted button controlls the contactor and gas flow, perhaps even the water flow if it has 2 selonoid valves. this hooks to the twist lock receptical, usually 2 prongs, but if there are 3, one is a chasis ground.
Closing the circuit should close the contactor and initiate gas flow. It may have a mechanical post flow timer which may or may not still work. One of My older machine sometimes goes through more than 1 post flow cycle before it stops.
Yours is the big brother to this machine:
http://www.millerwelds.com/om/o302j_mil.pdf I think they called it the 250 Twin or something like that, couldn't find a manual for it. If You find an old timer at a miller dealer, they might recognise it in a better picture.