Aquadoc, a couple of explanations about Dunhill come to mind. They used extensive nomenclature to protect patents, and to promote that the pipes had a patented feature that no one else had. Second, some of the letter shape stamps first denoted customers initials for custom pipes. Subsequent pipes could be produced if another customer wanted a pipe just like the one he saw Lord Throttlebottom smoking simply by looking at the pattern book. The initials stayed the same. Third, the pipes were guaranteed for a year, so the date stamp had a meaning beyond what it has come to mean to collectors. There may be other reasons, but those are the ones that come to mind.
Neat GBD'S, by the way. Love the sitter.