... I noticed that a countersunk screw was protuding out at the bottom of each slot which houses the seal, perhaps these are screwed in afterwards as well? ...
Rutger,
I have seen that a lot too. I think what happens is that if you start at the top and screw (4mm machine screw) each one in completely by the time you reach the last one at the bottom the channel is no longer aligned with the holes and it wont go into the conical hole. Better to attach all of them partially and then center them alternating one by one. I have also seen wrong sheet metal screws used there and of course then the thread is gone and a machine screw may not fit properly. But as Jordan wrote already wrote, all screws are screwed in firmly before you add the rubber seal.
