Not a double gun, but I once owned an H&R Model 999 Sportsman that had most of the major parts serial numbered. On a couple of them, the last two digits were reversed (71 instead of 17 - why would I remember that, anyway?). Since the stampings lined up so well, I always assumed that the group of numbers went into a holder and the holder had been dropped or the numbers had fallen out and been replaced improperly. Not that it matters, but Lefever wasn't the only company with this happening from time to time.