Could be the milling machine you used (looks like something I would do) Try a honing stone on it with a straight edge if it is only 1 MM. Cant find a machine shop to do it?
The manifold is very straight now. In the end I took off about 1/10 of an inch on the #7 exhaust port though, and I think that was a little bit too much.
I looked for a while but figured I could try this. A combination of laziness, curiosity and reluctance to reach out to people.