It is indeed a used MT EDC from 1973, it could be a good buy if it works and you need it.

I was wondering whether 9 29 73 3 meant that this dryer control is closing in on 45 years old.   So often we see a set of numbers and letters that require decoding.

Western Electric used a similarly obvious dating system on certain telephone components.  The last digit, when present, signified during which shift the part was manufactured.