Originally Posted by flo617
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.
From your diagram, you should only have to remove the vertical screws - that should let you lift the whole assembly out in on piece. You may have to slip a putty knife under the frame to cut through the caulking.
As for removing wood plugs - use an awl and punch out the middle, then working outwards, remove them bit by bit. Don't get greedy and try to take to much at one time. If you can't avoid damaging the edges of the plug holes, you can overdrill them the next plug size up - just make sure you don't drill deeper than the depth of the plug. A quick "depth gauge" is to put a strip of masking tape around the drill bit at the correct depth and leave the tape ends sticking out like a flag - it will sweep the dust away when it touches the wood.
I, myself, personally intend to continue being outspoken and opinionated, intolerant of all fanatics, fools and ignoramuses, deeply suspicious of all those who have "found the answer" and on my bad days, downright rude.