After looking after 50-257 for 5 years we are checking the door seals, windscreen seals, window felts etc. A mjr exploration and preventative job.
The door top panel aka window frame, on the interior side, has number 2 size phillips screws at the extreme left and extreme right. They are more or less inline with the window channel lip (and screw in, horizontal orientation).
Unscrew them and the window lock screws. Caution: "cracking" the first turn of the screws. These screws are likely tight and have been in place perhaps 30 yrs. Take care to use a good quality phillips screwdriver tip and press the screwdriver into the screwhead with plenty of muscle pressure (You DON'T want to roundout the screwhead). The threads of the screws are likely rusty too.
Now assuming you have those screws out, the glass lower edge channel panel still won't move - there are more screws involved and they are the 4 screws in the glass channel itself (they screw down - vertical orientation). Likely rusty heads and threads again... be extra careful with those heads, but when they are removed the whole lower window frame and glass wants to fall off the doortop. The window channel and the interior panel with the lock and the screw at each end are the one and the same assembly. I did not look at the parts drawing so this came as a surprise to me.
From that point you have access to change channel felts.
Take care with the lock assembly when its in bits - don't lose anything - they are very pricey locks.
