Very nice to hear from you, Mr. B!

This is an odd issue, and am glad I am not the only one running into it.
The new barrel is marked HV in front of the rear sight.

I don't understand how swapping the taller sight -cap for the lower one brought about a perfect zero. What are we missing from the equation?
I do see how a faster bullet may leave the barrel at a different point of the barrel flip, so may cause a difference in POI, but this much?

I will look at these different sight caps;
I wonder if the taller cap has less 'meat' on the underside, so though taller, actually sits lower?

The Commercial barrel came from N-Z, and is marked twice on the knox form, "K&H" as well as BSA & the stacked rifles logo. Any ideas on "K&H"?

Very best wises and come for a shoot sometime!

Richard.