I don't think they mean it in a literal sense, I take the expression "a right and a left" to mean that the horns come in a pair that are mirror image of each other. As campgems suggest in his application, it wouldn't be feasible to have two horns exactly alike, as, on one side of your plane the screws would be on the outside; and on the other side they would be on the inside. Like nature we have to keep things in symmetry.

Lm