re: the AC plug in the back of the unit
I wonder if this wasn't for connecting an appliance that might be used near the washer, such as a radio or an iron. Older homes, such as one that would have hosted this washer when it was new (<span style="text-decoration: underline;">not when it was already twenty years old in the 1950s</span>!!!), lacked plentiful electric outlets, so with the washer occupying an outlet, perhaps the AC outlet in back was to ameliorate the loss of one outlet. Analogous to mid-century electric ranges that provided an AC outlet or two (or one timed/one not timed) in kitchens that lacked enough outlets for all the counter top appliances produced after WW2.