Wiring AC ARC Welding machine Three Phase
It depends how you count phases. from looking at the back my guess is that you either connect 240V single phase to the 240V terminals  or a  380V supply to the 380V terminals.
I'm guessing that the welder has a single phase transformer inside it.
380V is what you get across two phases of some 3 phase supplies (where they use wye with 220V to a central neutral)
240V is a common 3 phase voltage in USA, but 240V single phase is also used.
Connecting to both sides seems wrong as there is a lot of metal between the terminals, and all conductors in a circuit should always pass through a single hole in the metal, also a welder doesn't need a neutral connection.