While all this info is well and good, and much appreciated, the thing is, I've done hundreds of mirror reverse components (at least), all done exactly the same way, and this is the first time this problem has reared it's ugly head. If it was something I had tried for the first time or if I had done something even a little different, that might explain it. But by now doing this is second nature to me, so all of the possible explanations offered up simply (IMO) do not apply. There's nothing different about this simple and very basic model that should or could explain that accursed white line down the middle of the steering wheel horn.
But I've put this behind me, and until it happens again I'll just add it to the ever-increasing list of weird things that happen in SU that you just have to live with