Drew,

That is a beautiful example of Alexander Henry's work. The factory records are available from Dickson and Son in Scotland. Here is the website:
http://www.john-dickson.com/home/welcome.aspx

They will provide the information that you request for a fee of about 15 pounds. I find it best to call them. They are very nice people to talk to and very helpful. I believe that the right lockplate was dinged somehow. There is a fine engraver named Bruce Bradshaw in Quebec, Canada who could fix your lockplate up right. He was an engraver for Rigby in England and he provides top work at a good price and most of all he does the work promptly. Here is his website:

http://www.bradshawgunengraver.com/index.html
Cheers,
Larry