I've never seen that stamp before. If you are really interested, email Jerry Hannah who owns the GBD webpage. He was helpful to me in identifying the history of my GBD's (two were a gift from a forum member, weezel).
His contact info is on this page:
http://www.perardua.net/pipes/GBDlines.html
His contact info is on this page:
http://www.perardua.net/pipes/GBDlines.html