I think you might be talking about the inside of the outdrive where the diff ring goes in. On the original XXX diffs, the diff rings had a "D" shaped cutout in the middle and the matching keyed outdrive. The new outdrives for the XXX CR diff do not have the D ring. They actually use a brass shim to help the diff ring slip a little and increase the life of the diff. If you have the new CR outdrives, just get the new CR diff rings and you should be fine. You can use the new CR rings with the old D shaped outdrive. It will still work.
Hope that might help.
TLR / Horizon Hobby / Spektrum / Tekin / JConcepts / MIP / B-Fast RC / UpGrade RC