I just realized that I wrote "head tube" but I of course meant "top tube", sorry.

I have two Merckx Corsa Extras, the cable runs through the top tube, not the housing. If the OP's bike has an internal tube that just fits the cable then it must have come from the factory however, I would be pretty surprised if that were the case.

As you can see from gomango's post, the factory job is far more elegant than what is in the OP.