Markings on a green WE 500

[AL_as_needed]

A friend of mine went to an estate sale and found what sounds like a dark green '62 we 500. Sadly I do not have pictures and the phone is still somewhere in shipping.... anyway...

With the model/date marking on the bottom of the base plate, there was a "U" in a circle stamped in orange. Now I know this may have been discussed here somewhere but I cannot find that thread. What does this stand for?