Don't take it as absolute, but I have been told the full octagon barrels were an attempt to use up some left over Hawken barrels - or to make up for the lack of PA Hunter barrels. I am not sure which twist they had, but if that information is true, it was probably the 1-48" twist. I do know that some - but not all - of the later versions with either barrel do have the QLA feature.
Here is some information I have found on the internet concerning the lengths and twist of PA's. Don't know the original author to give credit.
\\\\ PENNSYLVANIA HUNTER
50 (DEEP BUTTON) 28 & 21 Oct 1:48 RH
50 (CUT) 1:48 RH
50 (DEEP BUTTON) 1:66 RH
\\\\ PENNSYLVANIA MATCH HUNTER
50 (CUT) 1:66 RH
The octagon to round PA Hunter (rifle) barrel is 31".
Another change is the trigger or more specifically the trigger sear bar. The early versions appear to have a trigger bar that is split w/ the sear going between the two pieces - no spring. The later version is a solid bar w/ a spring. The early version has the tang mounted by two wood screws and the later has the front tang screw as a machine screw that comes thru the stock and screws into the trigger assembly.