Shut the door today and heard something fall inside the door. On removing the door card I found a black eccentric disc plus washer and nut. The bolt was still in the regulator mechanism and seems to be a means of adjusting the top stop position. Winding it up and down a few times it seems that this is completely superfluous as the window can't go higher than the grp in the triangle where the mirror is fixed anyway and where it touches the grp is just about the right height. The bolt itself is too short for the nyloc to bite which may have been the original builder using the wrong length bolt and is probably why it worked loose but having said that, if the eccentric was set to stop the window just before it touches the grp then it would be positioned such that it wouldn't take much force for it to rotate and lose its adjustment unless the bolt was done up FT. I've not refitted this top stop for now as it would appear that it has never actually done anything useful in the 16 years the car has been in use. Anyone have any opinion as to whether this is essential?

Guess, is that is the GTM top stop as described in the manual as '....fit...T521 to the door regulator using a 6mm* 18mm long socket screw and nyloc'?
and if its done very little for 16 years, get rid and think of the weight loss next time you are on track

Interesting Phil. The bolt is indeed an M6x18mm socket head. Two things strike me as wrong about that. Thing 1, once it has passed through the regulator arm, the eccentric disc and a washer there isn't enough thread left on an 18mm bolt to reach the nylon in the nyloc nut so is prone to working loose. Thing 2, a socket head seems a daft choice because the head of the bolt is only 1 or 2mm away from the glass so not sure how you are supposed to get an allen key into it to tighten it. I had to take off part of the regulator to remove the loose bolt. As such no way of tightening it insitu so not sure how you would ever accurately set the adjustment. A normal hex head could at least accept an open ended spanner insitu. I did wonder if it should have been the other way around with the nut by the glass but the nut is thicker than the head and would touch the glass.

As you say, it is now weight saved although I should probably remove it from the passenger side too!

I'm also hoping I've cured another rattle as if it has been loose for a while as I suspect it has, this must have been a source of rattlyness.

From what I understand the glass was fitted and then removed whilst the frame was fitted to the door, so I guess you would have had access at that point. I tried to adjust the Spyder windows, but never managed a good fit as the glass was in situ, so I lived with the wind whistle through a gap when on the motorway... or more often left the window open. Always something to puzzle over