The fixed sight 650 was a 3" heavy barrel and 651 4" standard weight. The 51s are a great gun and getting very pricey especially if it has the extra rimfire cylinder with it.
There isn't a lot of difference between the 3.5" and 4" barrels in shootability or packability...
Bought this one last year from the estate of a friend...it has the extra .22 rimfire cylinder with it...
This is a 63 with a fitted 651 cylinder....
There was also a run or 651s with 2" barrels....
Bob