Thanks Martin and Patrick for these replies. I'm very excited for the exhibit at the Landis Valley Museum!
The Jacobsburg rifle may very well be the only early Henry rifle signed on the lock and the barrel--but which Henrys made it and how "early"? It is, I believe, marked "J & W Henry" on the lock and "W Henry" on the barrel, so it has no connection, it seems very likely, with William Henry of Lancaster. The "J & W Henry" lock has to date from the partnership between William Henry II and his son J. Joseph Henry (or maybe even later, to the partnership between J. Joseph Henry and his brother, William III).
I agree with Patrick's point that there "is no concrete evidence that William Henry [of Lancaster] ever personally made a rifle." I'd go further and say there was no "Henry factory" in Lancaster (and no guns produced by any Lancaster "Henry factory" with the Henry name on them), despite the fact that secondary sources often state that there was. My opinion is that Henry had no involvement in the gun industry after 1760. He did not have a gun factory in the 1760s, despite what most secondary sources say. And, during the Revolution, he had no direct involvement in the gun industry (i.e., making guns or directly supervising others who did so), except as a financier and procurement officer--which is to say that his involvement with guns was no different than his involvement with shoes or flour or many other items. In fact, it would have been illegal for him to supply to the Continental Army guns of his own making while he was a procurement officer: this was considered corruption and others were charged and tried for such practices. (I realize this may be controversial. I try to make the case, with a lot of evidence, in an article that should be out in the Pennsylvania Magazine of History and Biography this summer.)
Any guns with "W. Henry" on the locks or barrels were made/used by William Henry of Nazareth, not by his father--at least that's what I think the evidence, so far, shows.
Scott