I know, frustrating isn´t it?
The way that I understand it is that pretty much all pre ~80´s Opels use the 6 x 6-1/8 x 2 HC (could be wrong though).
I think g.l.amour is using one of those, he probably knows better.
Do a search for "Opel" on the forum, there should be a couple of threads covering the Opel cores from a couple of weeks back.
Your best bet would most likely be to just figure out the dimensions of the HC your looking for, then go to the scrapyard and have a look around.