Your question is a variation of the one I just posted (I didn't read yours before I posted my question). I'm trying to add SATA drives and am trying to figure out if I really have to have an interposer card as well. I can help you with the part numbers though. The part number for the caddy that goes with the interposer board is CC852, and the part number for the interposer itself is PN939. I've also read that Dell won't sell the caddy or interposer board separately because they want you to buy the whole thing, including the drive, from them at a big markup. A 500gb SATA drive with caddy and board costs $399. However, if you go to the configuration utility and spec out a system, each additional drive of that type costs only $279. It's hard to understand why a drive would cost more when you install it yourself, versus having them install it. One would think it would be the other way around. That's what got me going on getting the parts someplace else.
This whole business of deliberately withholding certain parts so that you have to pay grossly inflated prices for something you could get elsewhere for about half the price, is really annoying. At the end of the day I'll probably spend more because of the time I'm having to invest in this versus just bending over the log and getting it over with, but at least Dell won't get rewarded for trying to hold me hostage.