I agree. "provenance" means history of ownership, regardless of how the ownership came about (except theft of course which doesn't result in real ownership).

If HM owned the rifle at some stage it makes no difference whether it was commissioned by him or simply delivered to him, he is part of the provenance. Just as If David Rockefeller owned a Rembrandt painting, he's part of the provenance of that painting.

In the gun world, I think provenance has a lot more weight if the prior owner was a famous shooter or hunter or outlaw. Fletcher Jamieson, Wild Bill Hickok, Jesse James, Bell, others. I'm not sure it carries equal weight in the case of royalty.

However, there may be UK royalty afficionados who are not even gun people who would value this rifle highly, assuming the provenance is real and documented. Photos of the man on a hunting trip with the rifle, etc., written accounts of a hunt, etc. would all enhance the value.

It will be interesting to see. My guess is it won't fetch the asking price. But I don't blame the owner for trying.