DDD and MD - Mechanical Doubling is more common than true Hub Doubling which would be a Doubled Die Look at this entire website - http://doubleddie.com/144801.html
I have an unrelated question I'd like to ask ya seeing you seem to know what your doing. I just purchased a sealed 1957 proof set. Should I steam it open or should I leave sealed??
If it is sealed, it is original/not tampered with. It is possible that the envelope was opened over time, and it is fine to open that, but not the cello wrapping.
Oh ok kewl....thanks for help. I think I might just try and steam it open, I would like to keep it intact..