According to what I was able to find it depends which motor the machine was orginally equipped with. North American or European voltage requirements. Here is the info. from a website I found.
"Motors: Models 221 and 221-1 were originally equipped with 110-120 volt motors. Model 221K Featherweights were originally equipped with either 110 volt or 220 volt motors. Except for the motors, the mechanical components of all Class 221 models (221, 221-1, and 221K) were fundamentally unchanged from 1933 through 1961."