In the Manta there were two types of vents. The older one is fixed with 3 bolts and a the last version were only pressure fixed!
I think that in the Ascona is similar!
http://img95.imageshack.us/img95/9434/saidaarvg7.jpg
http://img171.imageshack.us/img171/8...raseiraxn3.jpg
http://www.opelgt.com/forums/attachm...114-small-.jpg