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I have often wondered this myself
I have asked many folks this question and for the most part have always gotten a simple and short explanation: A: "Nothing".
The empty weight got heavier over the years as they added more interior bits, made changes to fuel system set up etc, some change to gear power and associated plumbing, but to my knowledge the structural part of the airframe never changed much at all (some change early on to the chord length of the elevator and trim tab size and travel as well). Also some change to the flap operating range.
As far as I can tell, all the changes to the model appear to have added empty weight and as they went up with that, so did the gross weight (certificated). Of course power has always been 210 HP each end for the NA machines. This leads me to believe that the lower gross weight of the early models is purely a paper issue (ie this is how they were certificated). I have heard folks who have flown lots of different skymasters (not me) express the view that the early planes climb better (I guess if both a later and early plane were both operated at their respective gross, this would make sense). It seems to support the view that structurally the machine is the same through the years.
Don't take this as the definitive word, though, because I am no expert. Don Nieser at Commodore Aero has taken many of them apart over the years, so maybe he knows for sure.
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