Well, I got out my kit (it's in my "not to be built anytime soon" stack - but that's another story best not brought up again) and took a look. The Tom's Modelworks photo-etched set that I bought for it (#364, "Destroyer and Escort," probably not currently available) contains SA, SC, SR, and SC-2 radar screens, all of which, as I understand it, were carried on the masts of American destroyers at one time or another during WWII. The plastic part included with the kit (and shown on the instruction sheet) appears to represent the SC screen. The same sprue also includes a part that looks like it's intended to represent the SC-2 screen, but that one isn't shown on the instructions.
There are several potential sources of miscommunication here. I'm assuming we're talking about 1/350 scale. (Dragon also offers a Gleaves-class destroyer in 1/700.) The Buchanan kit I have is the 1/350, 1942 version - kit #1021. Dragon also sells a 1945 version of the same ship. I imagine the SC-2 screen is intended for that one. (The Gold Medal set also includes SC and SC-2 screens - as does the Gold Medal set for Fletcher-class destroyers.)
Constructor - is it possible that "SC-12" in your last post is a typo for SC-2? If so, it looks to me like the etched part you've got would be appropriate for the 1945 version of the Buchanan, but not for the 1942 version.
This is pretty picky stuff. On the other hand, it's such a superbly detailed kit that the finished product will invite close examination; I can't blame anybody for wanting to make sure it's right.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.