What I gleaned from "The Car Bodywork Repair Manual" published by Haynes,
ISBN 0 85429 530 5 ...

"...some screens of this type have the additional feature of a rubber
spreader strip which is let into yet another slit in the front of the
windscreen rubber. This has the effect of pushing the rubber into closer
contact with both the screen and glass. It can be fitted using the special
tool shown, or a more long-winded alternative is to use a pair of
screwdrivers: one broad bladed to open up the rubber and one to push the
filler strip down and into place. Wiping washing-up liquid over the filler
strip makes it considerably easier to get it into position."

The special tool looks quite simple. It consists of a handle from which an
elongated closed loop projects. Behind the loop, coming out from the same
handel is a roller. The loop opens the slit and ypu feed the trim in and
it is forced down by the roller and held in place by the rubber when the
roller rolls of it.

The loop comes to an acute point wher it is to insert into the slit, then
widens to form "wings" (with dihedral), then closes again to go into the
handle. The roller look to be postioned about 1/2" behind the loop
insertion point, but I think this distance can be adjusted (set screw in
the handle). The trim comes down from above into the spread slit between the
loop and the roller.

My apologies for the awkward explanation.

Alistair