The set I bought years back had a bevelled rubber ring for use between the housing and the retaining collar. As mentioned by tclederman, there are four paper gaskets which help to determine how much spacing is needed between the collar and the transmission. It is indeed very well described in the shop manual. The advantage of the individual paper gaskets is that if you have leakage in the collar area sometime later, you can loosen the four bolts and rip out one of the gaskets, making the joint tighter. They might seem thin but they are effective and there is a difference in movement of the housing when using one or four.
