According to DeWitt Bailey, 'No English locks are known and dated after 1764 when the process of dating locks was officially abolished'. There are Irish Dublin Castle locks marked 1767 & 1769. Once again, that's pre- 3rd model. My interest is American war periods, so I can't comment on whether the practice was re-instated or, it could be possible that an individual regiment could mark and date their own locks?
Sorry, This isn't much help.
Ryan