I would think that if the FD is original then the rear was probably Cyclone as well because no manufacture that I have ever seen ever put a lesser quality derailleur on the rear vs the front. ARX isn't a bad derailleur though, I have one and it works fine, but the Cyclone was one level up from that one and has a solid reputation for reliability and quick shifts not to mention a bit lighter. Not sure why the rear was replaced, maybe the owner laid the bike down and broke it, or it may have had a fluke part that broke and it couldn't be fixed, but it must have happened shortly after the bike was purchased but after warranty because ARX was made the same time period as the Cyclone Mk II was being made. Word of caution when servicing the ARX is to be real careful with the cable clamp bolt, they had a tendency to strip, not sure if a better bolt could be found to replace it, I haven't had any issue with that bolt yet but it is prudent to be careful when dealing with it.