If this is indeed a conversion from a muzzle loader why did the muzzle loader have barrel flats?
I have a Barber and Lefever conversion that looks very similar with English proof marks on the barrel flats. It was recently restored by Buck Hamlin. He explained to me that it was not likely to have been a conversion from a muzzle loader, because muzzle loaders don't have flats. He tought that it was a redone breech loader. That could have been due to a loose or broken frame on the original. Buck claims thats not all that unusual in that the very early breech loaders were not all built as well as the Sneider or Lefever conversions.

Great-Great Grandson of D.M. Lefever
www.lefevercollectors.com