I found an equal-length 6-1 header for a square port yesterday. Which companies make these & how do I tell the difference?

If there are no identifying marks stamped on it, there is probably no way to tell. In the 35+ years since the Z was introduced, there have to have been dozens of different headers built, many by small companies that no longer exist. Good luck!

I guess it doesn't really matter. The construction is pretty good. A lot better than some other headers I've seen lately & for $22 HAHA! Thanks