I had my vintage MBW 1665 apart a zillion times and never had the gasket that goes beneath the crystal retaining ring, into the groove in the case.
I just smeared a thin film of vaseline on the outside of the rehaut and pushed the crystal down, twisting it a little, making sure the vaseline squirted out evenly all the way around. Then I smeared some on the outside of the crystal "wall" and the inside of the retaining ring, and pressed the ring down snug. Of course I never went very deep with it, certainly not more than 100 feet, but it never leaked a drop.
Moral of the story: you might not need that gasket. Plus with the world ending in 3 more days... just sayin' ........