2456s (the 18K Carrera 12) are pretty rare indeed. I have seen, either in the metal or in photos, maybe six or seven distinct Ss, but if you want really rare you need to look for 2456 Tachys. 2, maybe 3, separate late models with the black scale but I've only seen one 18K watch with the early red scale.

So if I had to pick one really really rare Carrera, a 2456 T in first execution could well be it.