If it's anything other than a design element it could be the designer's initials. A lot of coins have those somewhere small in the design. I don't think it's a mintmark.
It is a mark to tell you which die was used AA being the first die AB 2nd and so on however AD is not listed :S
Here is some info (which you may well already know ) about those die letter combinations: http://www.wbcc.fsnet.co.uk/sub004b.htm But I don't think there is a final word about what combos exist ... Christian
