Doubled Die Question

So looking at this pic from the philly mint it looks like the hub and die have slots to fit into, So how could you get a double from the hub to die if it is held in place with the said slots?

 

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Not going to type all this so go to Wexlers site and read all about it.

 

so they put the lugs in place after the 1955, then took them away until what 73 then put them back? So does that mean there has not been a BIG doubled die since 72?