In my years, I have seen just two such barrels. One was on a fine Henry Albright, Lancaster rifle, which had an inch wide insert, dovetailed into the barrel somewhat ahead of the breech on the left quarter flat. It was stamped with what may have been the barrel maker's initials. The owner theorized that the maker had forged the barrel, repaired the flaw, and signed the repair to warrant that it was good. Interesting gun, and the reasoning on the purpose of the initials on the repair seems sound.
The other piece is one that is hanging up on my wall. It is a J. Bonawitz smooth gun and it has six small inserts dovetailed into the left quarter flat of the barrel located about half way down the octagonal portion of the half round barrel. They cover what seems to have been a rather wide flaw judging by what is visible of it.
Guess that they wasted as little as possible back in those times.
Dick