Waqas,
now I'm starting to doubt myself. Fairly sure in my car that the channel is not simply a u-shape, but is a square "u" with a lip that hangs back over the channel, as drawn. My proportions may be off, but that's how I remember it. Car is a 280 SL. The flat part of the seal goes against the wall between the channel bottom and the lip. If you think of the seal cross-section as an "L", the L is on it's side, with the long leg of the L on top. I'll try to double-check next day or so, but I installed my new seal just like the one I took out, which I'm guessing was the original, and it popped in beautifully.
George Davis
'69 280 SL Euro manual