The unit does have a 115v tap that is used to power the external wire feeder but nothing internal uses 115v. There is no hook up for a neutral that I can find.
Page 26 of 38 is the WD for the unit. It does not have the spot weld function.
http://www.millerwelds.com/om/o1539c_mil.pdf
I'm correcting the PO wiring for the input power cable wiring on my old Miller Spoolmate 200 (same power supply as MM200) but I've got a question. It's pretty obvious where the two hot wires go but since the machine only used two conductors back in the day and my garage is wired to current code...