DS window removal 1969 Valiant

That plastic rivet is subject to breaking. Aftermarket offers a replacement ( RT Specialties I think ). I use a piece of neoprine fuel hose and a 1/4 X 1 inch bolt with nyloc nut.
The glass sits in a steel sadle ( shown ). That sadle should have a gasket material linning between glass and steel. Seeing the entire linning is gone suggests that glass has been out before.
Glass removal for post and no post models is a little different. A factory service manual will be a great help.