It is a **** shoot. Unless you can remove the cover who knows what you have. Just purchased a Miller xmt 350 mpa, lucked out as the seller had a lightly used unit just tested/cleaned/calibrated. Major issues with well used units is how old and how much grinding dust/metal particles are inside and whether unit was subject to repeated condensation. Many times you can tell how much use/abuse a unit has had by carefully checking out all pics.