As far as being "correct" to the 18th century..., post American War of Independence, not bad, first decade of the 1800, no worries.
While it is true that it doesn't match the current standards for a Southern Mountain rifle as the lock isn't as "English" as those found on surviving examples..., it is rather tough to pronounce such a lock as "wrong" when perhaps we should be saying "not the best, but not bad".
This is of course my opinion, but so few of the original rifles of any school remain compared to the records of the number that we can say were made, who knows when different locks were substituted? Who knows how many rifles had locks scrounged as used from other guns?
Below is the link for a paper on the 1792 and 1807 "Contract Rifles" ordered from civilian makers and bought by the US. The government standards were issued prior to 1792, and first deliveries and accepted rifles happened in 1792..., hence the terminology. Out of 3400 rifles bought, less than 2% survive, and the example in the paper is a flinter converted to percussion. The lock plate is considered original. It is very similar to your lock's plate, only the tail of the lock plate in the original was bobbed. Otherwise the lock plate lines are "Germanic". Which shows that such a lock style with Germanic lines were being used in the colonies, though maybe not in the South as "new". It appears that all of the contractors were from the Lancaster area, so that would explain why they had Germanic style locks on some of the contract rifles. Again don't forget about the possible use of a used lock by a Southern gunmaker. There would be nothing to stop a Southern gunmaker from using such a lock, just that the availablity would probably have been less than one of English lines.
If you check the paper further, there are three 1807 contract rifles shown, two with very similar lock plates to yours, one very different. So with that in mind, my opinion is that the lock on your rifle isn't "bad"..., it's just not the classic type of lock many folks expect to see on such a rifle. It is, however, within the realm of possible locks that might have been used on such a gun prior to 1800, and after. Again it just depends on "how" a maker crafting an iron shod, mountain style rifle came to use a Germanic style lock when it appears that the English type was more available.
1792 and 1807 Contract Rifles
LD