Getting the number off the motor is the most common mistake people make with these. Then the seller gets upset when they were convinced they has something really old - when instead they have something rather common.
Some really early electric machines came with a cord that you screwed into a light socket - since outlets may have not been available. The house I grew up on - had no outlets in the upper bedrooms- just one light socket in the middle of the ceiling. It was built around 1926 and the upstairs was never updated.
BTW - many old machines got updated electrical connections at some point, so you don't find a lot with the really old type. Just wasn't safe if you wanted to use the machine. I'm all for "vintage" but not when it comes to electrical.