They have good drawings that should be of help:
https://www.sls-hh-shop.de/en/mercedes-230-280sl-w113/72-door/72door-windowThe way I did it (not sure if the best) was to fit the front and rear rails, but leave the screws lose. Fit the lifting mechanism, again, not tightened. Slide the glass with jaws from the top along the rails with plastic inserts, plastic washers and adjustment screws fitted (lose). Than adjust and tighten everything as per adjustment procedure.
The lifter rails have felt inlays where glass touches them. As my felt perished, I cut rubber inlays. There are rubber bushings going through the glass where screws go.
Plastic inserts go into jaws (plenty of them, different ones - left and right, front and rear and one for the chromed corner front top). On one side they are fit to glass with a adjustment screw, on the other they are held by plastic washer stuck on the small stud in the insert.
"Attachment screws pass through plastic spacers" you wrote - I am not sure which spacers or screws you mean. If you mean the plastic inserts and adjustment screws, no, they do not pass through plastic anywhere.
For me, the most dangerous moment was tightening (slightly!) the lifter rail to glass. Needs to be done gradually, delicately, the screws are not really tightened, it is a "light snag" fit.