CCM, as I know the company, is always making little tweaks to their stuff. THe little things are many - the end product shoots the same (valving and the like).
THe difference between the first S6's and the latest are a few:
1) There was no auto trigger retention system on the oldest S6's. They added a ball and spring to the cam for a time and now they come with an Auto Trigger retention rod.
2) They changed the milling on the portion under the barrel so that more barrels could be accepted and the front end strengthened.
3) The changed the milling of the hammer slot (bottom of the body) to prevent any mushrooming of this area.
4) They made the VRA integral to the body of the marker.
5) They changed the style of the cocking rod.
I am sure there are other little tweaks...