Geppetto,
IRS must have changed it, because my friend just bought that IRS 1/12th scale diff kit for his brand new RC12L3 and it includes D rings and the cool machined left side hub as well. Call Irrgang and make sure the part number is correct and it is now for D rings.
www.stormerhobbies.com has a good selection of IRS stuff for a good price. Let me know how it works out for you.