So, I've explored a bit more and I think I got it now. The fasteners seem to be on the outer side and were hidden under the rubber that was used as a gasket. It seems there are two per side. The horizontal fastener on the inside are probably keeping the inside part of the frame connected to the outter part. I tried to draw a little diagram of what I think is going on.
I wonder if I'll be able to extract the plugs without too much trouble or if I'll have to grind them down to the screws.