[Frecklychimp]

If it hasn't got round diff rings i doubt you have a XXX CR, so buying parts for a CR will cause compatibility issues.

If you have the white 2.43:1 diff gear then you should be able to just buy a set of XXX CR drive rings (LOSA3039) and use the 'wrong' outdrives you have already bought as the gear is the same in gen1 and gen 3 gearboxes

other than that you need a pair of LOSA3073 outdrives, you should already have the aluminum tube insert that converts one from female to male.

D shape diff drive rings are LOSA3070

avoid anything other than part numbers i've listed above as they will be for other models/variants. Gen 2 is for the XXX BK2 which has a grey gear case and 2.56:1 internal ratio so has different diff parts/outdrives