Molly,
that is a fine rifle. I love the patchbox and the toe plate.
I have no knowledge of Sites whatsoever, so with that in mind I have a few observations and questions.
Was he a builder in the flintlock period?
Looks to me like the lock may have been converted to percussion. The engraving on the lock is cut off by the cut-out for the nipple drum, indicating a possible conversion. The hammer doesn't seem to be square to the face of the nipple, unless the gunsmith angled the face inside the hammer to match the nipple.
Looks like the lock fits the mortice well, I would assume that the bit towards the bottom of the lock in which you can see the mainspring is just wood shrinkage, rather than an indication that the lock was replaced. Added to that, is the previously mentioned idea that the it is a conversion.
Looking at the side panel created some confusion to me. It has a single lock bolt, but yet I think it is a conversion. Late period maybe? Again, I don't know Sites. Also, the black shaded/looking areas of the side panel almost look like the impression of a side panel inlay. Added to that, when I zoom in on the picture, it looks like there might be a wood plug of screw diameter exactly where I would expect to see a front lock bolt. Is it just me imagining? There is no evidence on the lock of a long gone front lock bolt.
Anyway,
take these musings for what they are worth. I do not in any way intend to impugn the history or value of your rifle. I am just attempting to further my abilities to look critically at original guns and would like to hear what you think and see with your rifle.