They’re for something called Cro-knit, cro-hook, and crochet on the double. I’ve played with it a little.
The way it’s worked is very similar to Tunisian crochet, but the fabric it makes is essentially reversible (though one side is more one color, and the other is predominately the other color.). The fabric is a bit stretchier than regular crochet or Tunisian fabric. As I remember – it’s been awhile since I did it – you work with two balls of yarn at a time; one from each end of the hook. It really looks kind of neat.
There are hooks that are long like knitting needles with hooks on both ends, and ones like you found with the cable – the cabled ones would be for larger projects like afghans.
That’s all I know about it. :shrug:
Edit: Come to think of it, I think I have a swatch of crohook crochet around somewhere. If anyone’s interested, I can see if I can dig it up and get a picture.