Rob,
The stock architecture is off from the typical Bedford/Stoudenour (i.e. little too fat at the butt, absence of lock panels, shape of cheek,..). The engraving is not in the style of Stoudenour. The side plate is not a typical Bedford county side plate. The patchbox is a typical of Bedford county patchbox (may be Somerset County). The lock plate is not a typical Bedford county (rat tail is wrong shape). The turned trigger is not typical of Bedford County (may be Huntingdon County). Additionally, the hammer, lock bolt and drum all look to me modern. And maybe even the lock plate?
Here is the list of makers with the initials "J. S." from "Gunsmiths of Bedford, Huntingdon, Fulton, & Somerset Counties" by James B. Whisker and Larry Yantz (Track of the Wolf, 2001).
Jacob Saylor
Joseph E. Sivits
John George Slonaker
Jacob Sloop.
J. E. Smith
Jacob Snider
James Stapleton
James M. Stapleton
Joseph Stapleton
Jacob Stekenius
Jacob Stoudenour
John Stoudenour Jr.
I am almost certain this is NOT a Snider, Stapleton or Stoudenour. I have handled many example of each.
It could be a period restock or a conglomeration of parts.
Why do you think it is a Stoudenour?