Excellent details VicKlicktor! I take it the red clip is to use when the coupling is not in use. I have some pictures of the older and newer style couplings which Playmofire sent me.
I wonder if other rolling stock sets with more than one version also have this difference in the couplings? 4101, 4104, 4105, 4107, 4110, 4112 are all candidates as, like 4100, they have 1 or 2 earlier versions from before 1987/88 and a later version from 1987/88 to 1994 (according to Collector), though you say 4105 doesn't have the later couplings in the later set?
It might not be that simple anyway. Versions 1 and 2 of 4053 (1986/87 to 1992/93 and 1992/93 to 1995 respectively) have the older style couplings but version 3 (1995 to 1996/97) has the newer ones. Perhaps the change from older to newer couplings was not aligned with a change of version, but just happened when the newer parts became available. I don't know enough about how the two couplings work to know whether there were minor changes to the undercarriages to accommodate the different couplings. In fact, I don't know much about PM trains, my interest is really in trying to pin down the differences (if any) between the contents of different versions of sets, something which is quite lacking in some sources.
I don't think the different pipe colours in 4105 are significant enough to describe as a change in set contents between the versions, but it's interesting to know about!
Thanks again, keep up the good work. I don't have many boxed sets, so I rely upon people like you to show what's in 'em!