[folivier]

On my 1998 #486 I had the same or similar problem. Yes those 2 tabs are supposed to lift the slide slightly to make a good seal at the top when the slide is extended fully. After lots of head scratching we figured out that sometimes one or both wouldn't engage and the tab would go under the slot pushing the slide down creating a gap on the top of the slide allowing rain to leak in. Tom and I tried adjusting the angle of the tabs after we removed the long bar but weren't successful. Ended up going to Newell and they were able to get it adjusted correctly.
Once you remove the valence you'll see the bar and 2 tabs/slots.
Good luck!