Well, in pre-Scratch Alternia, Karkat would have been the Signless, right? Since he and his friends were the Ancestors in that iteration of the blue universe. And then whatever important symbolic event happened to him would have informed whatever sign whats-his-face eventually got. Which is probably the Irons, because yeah, shenanigans, and also Aranea wears it as a necklace so clearly it's a piece of iconography that exists.