jamned,
I must respectfully disagree with your assessment that this is an original John Shell rifle. As a descendent of Martin Shell, John Shell’s father, I’ve viewed and studied many rifles made by the Shell family of gunsmiths, particularly John Shell, the most prolific of the family.
Based on your photos, I believe you have a restocked rifle with some John Shell parts.
The barrel is likely a John Shell. The signature is similar to that of other John Shell barrels, though it appears to be missing the distinctive border usually found around his barrel signatures. Maybe this border is there or has worn off– I can’t tell from the photo.
The entry pipe and trigger guard may be John Shell’s work as both are similar to those on other of his rifles. However, that is questionable as both are lacking any engraving, particularly the engraved “star bursts”, commonly found on Shell’s rifles.
However, the patchbox is not John Shell’s work. The finial is unlike any I’ve seen on his rifles, both in terms of design and being attached with pins/brads.
The side plate is also unlike those found on John Shell’s l rifles. And it’s missing the number of the rifle, a practice Shell used on his rifles.
As you noted, the lock is a replacement. The single lock bolt would have been consistent with a percussion lock.
While your rifle is a restock, it’s a fine example of the long use and reliance on it by its owner. And it’s testament to John Shell that the owner would want to re-use the John Shell components they had on hand.
Bill