Hi Sweany,
just for clarification as I can't match your elaborations with the shapes of the parts in my collection (as far as I can remember):
Many of the parts I have are from the time of the old 50/1 100/1 boxes and as far as I recall they didn't have the extra holes. The "newer" parts I have are for example from the "Motor Master" (
http://www.fischertechnik-museum.ch/mus ... m=52&pos=3) - which apparently is from 1991 and as pictured in the manual don't have the extra holes either. And in the really new boxes such as the TX PneuVac, the parts also don't have extra holes for sure.
So to me it looks as if there was a time "in between" when the parts had extra holes and this intermediary variant was abolished again.
Therefore my (uneducated) answer to your initial question would have been: "No, the red part is not a new variant, it existed for some limited time after 1981 (only)" - and with some insight to the ft-comapny archives (which as a regular user I don't have) I also would have told you when it was abolished and why. I'm convinced that was quite a while before 2015.
From an outside perspective, I can't imagine that something compex as a casting mould would be exchanged accidentialy without anyone noticing. (This certainly is different for the materials used for casting, therefore I, too, would have hoped for some more detailed insight on the colour issue, but thats something different)
And for my personal education: What is that hole good for anyways? I understand you can slide a "Verbindungsstück 15" (31060) in there, for example, to lock it and hinder it from moving out sideways. On the other hand, the hole there weakens the part so I can well understand that it was removed (again).
You referred to models that can't be built without the hole - which ones? On a quick search I didn't find any but I'm always keen to find out new applications for the parts.