Yes indeed, it seems the 50 ohm is always in series with the 1M input.
But I also think the 75 and 50 ohm impedance corrections are just set in parallel with the
1Mohm input directly on the BNC input when turned on. So the measuring circuit stays the same..
And I agree that the protection circuit is just behind those roasted 50 ohm resistors.
Since the scope works well again, that protection circuit is at least not short circuited.
But I wonder if it has survived and if it will still protect my scope when things go wrong.
I can't find any other roasted components... So it might be okay...