Well there's really no use in debating the issue based upon photos alone. To make such determinations with real veracity, it's often necessary to have the piece in-hand.
It is quite possible to plug and camouflage a percussion drum hole and realistically age it to match if you know what you are doing. Fact. Would need an x-ray or bore camera to verify.
Whether or not this lock was original flint, the cock at the very least looks to be 100% Siler and I suspect some of the other exterior components are replacements also.
About the engraved lock plate line: I take your point but I have seen quite a number of guns - some very nice - which were built as original percussion guns using locks made from older flint lockplates. At least a handful of them did not even bother to plug the exterior spring/frizzen holes, no s**t.