I assume that the letter says that both were a short rifle with shotgun butt? With one having a factory serialed rifle butt and plate it is obviously a factory error made in the log book.
When and where the stocks were switched is still unknown. But as long as you have the letter to show that both were logged as short rifles I would say you are okay. Would have been nice to know if both SR's were shipped to the same location. If they were the stocks might have been switched at the store. If not then probably done at the factory.
Just my $.02 on the subject.
