SD - Strike Doubling.... just another, possibly older term for Machine Doubling. Maybe I am a little off in my usage of proper terminology. I am still stuck on "rotated reverse" as opposed to a "rotated die", but that's a whole different story. I would like to comment on what the gentleman said about there being many other significant doubled dies still out there that just do not get the attention that, for example, the 1955/55 does. He is exactly right and I personally believe that it is a sad thing. Just because a certain variety is not well-known, listed in CherryPickers, or wherever, that does not make it any less special or possibly significant. As for the photos of the reverse, I would say it's DDD (Die Deterioration Doubling) and not MD/SD.