The retainer ring has to fit down over the top of the EDIS adapter, line up with the six bolt holes, and provide a means of securing the tec3 gear in place.

The regular EDIS toothed gear is alloy, and sits flush with the top of the adapter: because the adapter was designed to do that. The six bolts would secure the EDIS gear easily. The tec3 gear is steel; and much thinner. It is 1/8in steel.

A retaining ring was required.