Bill, I commend you for your LONG ongoing search for this gun. After my weak attempts to find it, I am convinced that some Wistar family member does possess it, BUT WANTS TO KEEP THE OWNERSHIP & LOCATION A SECRET. I hope you can prove me wrong.
Note that the 1896 published genealogy of the family indicates that the family believes that this specific gun with its rather long barrel is the one brought from Germany in 1717 (which I doubt). By 1896, the trigger and lock were missing.
Note also that the Hagley museum has info in the estate of Caspar ( as well as great info on Henry's Boulton Gun Works, Kriders Philadelphia gun store, etc). Unfortunately, I do not live in Wilmington, Delaware any more and can't avail myself of the museum's vast collections