Over the years I have seen many of those old Nautilus filters operate without the "O" ring you mention and not leak DE into the pool. Try cleaning the pipe and inside the manifold port and making sure that the assembly sits as far down as it is meant to. Check for cracks in the manifold, especially around the holes for the rods. Make sure that the air-relief screen is in place and not damaged.

Are all of the grids properly seated in the manifold?

Another common cause for DE to be in the pool is worn seals in the backwash valve.

Hope this helps.