Hi,
When you remove the end pieces, there are two M8 (or M6, I can't remember) screws below that on both ends. Bavarian way is that there is metal plate, most likely aluminum, laminated into the roof and there are threads where the screws are bolted into.
And since it is quite old boat and you have aluminum and stainless steel, the screws are most likely corroded quite tightly. Mine didn't come cleanly off and I had to use quite wide arsenal of tools to get job done.
Good luck with that.
Jussi