The lock plate is similar to a contemporary TC or CVA Hawken, but due to its age it's hard to know where it came from without taking the lock off and looking for some clues.
Are there any markings on the gun at all?
Very early CVA flintlocks had double triggers that looked similar and I think that they were assembled in the USA using a combination of domestic and imported parts.
How do you know that your gun was a custom and not ordered from someplace like CVA, or from a builder that used CVA or imported parts?
To believe that the lock and/or barrel was switched from flint to percussion without some tell tale signs of fitting the wood around the lock mortise would be a lot to speculate about from simple photos of the exterior.
How long is the barrel? Is it a long gun or a half-stock?
I believe that early CVA flinters were more often .45 long guns that were made with Douglas barrels. They looked custom, could be made to be extemely fast and worked with precision.
That lock mortise looks original and made for that lock. The drum looks as old as the gun. So IMO from the photos, the lock and drum don't look like replacements to me.