OD = Own Design. I've also heard "our design."
A-J = Grade (ascending)
So your pipe is an Own Design, Grade B (higher than "A;" B's are typically very big, thus the briar costs more). If I recall, the pipe is fairly old... early fifties. I'll have to read the page TopD listed, as I've not seen that one before. This is from memory, so I might be wrong. My friend's dad is a Dunhill fanatic and we've shared many a bowls examining his collection. Well, me drooling over it, anyway...
Thanks for providing that site, TopD.