Gotta admit to not a whole lot of experience with Heinie's sights, but do know that the rear sight on the Defender and C.C.O. is an Ira Kaye-designed monstrosity that I seriously thought had its own weird dovetail cuts. While vaguely similar, I don't believe the two are interchangeable. Why anyone, either Kaye or Heinie, would create such a hogged-out design is way beyond my limited comprehension.
Rumor has it that Colt's received so many grumblings about this rear sight design that they're contemplating offering the Defender and C.C.O. slides on an "uncut" basis. Then again, this makes sense, so we probably won't see it happen.