Mike,
I have an 1861-dated Pattern 1853 Enfield rifle musket with a mark in the identical spot you describe and it "may" be of the same type. I can't tell for sure from your various descriptions without seeing an image of the actual mark.
If you send me an e-mail with a close-up digital photo of your mark, I can tell you if it is the same mark as mine. If so, I have a theory as to what it may represent.
Mike