Period "Spare Parts Kits" that were sent along with British Regimental Artificers/Armorers (especially for Regiments going overseas from the Home Islands) in the 17th and 18th centuries always included spare Lock Parts such as Steels/Frizzens along with tumblers, sears, springs and screw stock, if not actual screws and other parts and materials.
However, we do not know a lot about those parts and even where they came from, for the most part. Speculation is that they purchased spare lock parts from the same Lock Contractors who provided complete Locks to the Ordnance Board. Most of those parts most likely would have been in Die Forged, though unfinished condition. Using spare parts forged in the same dies used by the Lock Makers, would have made it much easier to hand fit spare parts, when truly interchangeable parts were still decades in the future.
Of course there is no way to tell for certain who fit the replacement Steel/Frizzen to this lock. It MAY have been done by a Regimental Artificer or it may have been done by what was commonly referred to as "A Country Gunsmith" in the geographic location the Musket was used and the Steel/Frizzen broke or worn out. I do not believe the Steel/Frizzen was replaced by the Ordnance Board or Lock Contractors who were paid to overhaul/fix worn/broken locks by the Ordnance Board, as it is a safe bet the Steel/Frizzen would have been engraved with the "border lines."
It is fascinating to see repaired locks like this and speculate on who fixed it.
Gus