In the Horus Heresy artbooks, theres a picture of him with some termie body guards, and on their un-preheresyish shoulder pads is the crux terminatus. Is this just the artist's failure to know that the crux terminatus not around yet, or is there something about primarch bodyguards or something?

? Are you talking about the picture labelled Santar? That's the only one I can think of that has that.

The artist is a spoon, I'd say. You're better off going with the picture opposite at the bottom, though they aren't Morlocks, they're just normal termies. The Morlocks' armour was gunmetal.