When the US Government started altering guns to percussion via the "Belgian" or "cone-in-barrel" method, they chose to do so because it was cheap. In fairly short order the Ordnance Department discovered that m1822/28s altered that way were unsuitable for rifling because the barrels were too thin for the cone to safely remain seated when the guns were fired with conical ammunition.
In 1855 the Frankfort Arsenal began altering guns with new breech pieces and new made Maynard primer locks manufactured by Remington. These alterations were vastly superior to the old style and could very safely be rifled and handle the increased breech pressure of conical ammunition.
By that point the US had a huge pile of percussion altered muskets and didn't really spend anymore time or money modernizing them after the ~20,000 Remington-Maynard alterations were completed in 1858. When war broke out in 1861 most of the leftover arms that were made after 1820 were altered to percussion, and the various States that had stocks of flintlock arms altered them as well. The chambered breech method used on the Remington-Maynard alterations was the preferred choice for most of the alterations done in the North since it is manifestly better than the older cone-in-barrel style and they had the industrial base to do it. Some fairly decent numbers of previously cone-in-barrel altered guns were realtered with chambered breeches as well, but finding exact numbers is mostly impossible since the contractors don't seem to have differentiated in their logs or charged different prices for realterations.
This Butterfield alteration is one performed for Pennsylvania, and as the contract originally specified was supposed to have a patent pellet primer mechanism on the lock. However, Butterfield took too long in delivering the arms (and his mechanism wasn't real great to begin with), so his contract was amended in 1861 to deliver the balance as standard percussion altered muskets using just the new breech pieces and hammers from his prior work.
If you intend to collect percussion altered arms you should really get a copy of George Moller's "American Military Shoulder Arms, Volume III" and Murphy & Madaus' "Confederate Rifles and Muskets".