I've never seen a reference to an exact date when the metal rondell was first used. Below, from the Pipepedia page seems to indicate that it was in the 1920's.
http://www.thepiperack.com/blue-chip-unsmoked-pipe-1920s-gbd-xtra-french-hallmarked-billiard-w-box-and-sleeve/
Having said that, hallmarked GBD's from the 1920's pop up infrequently, but I can't recall seeing one of those with a metal rondel,but rather a stamped rondell like this one.The French GBDs more or less followed the same developments, although Xtra and Speciale very longly used there. In the late 1920s a GBD with a metal filter system was introduced under the name Extra Dry. Also from Paris came another important new feature: the introduction of the inserted metal plate with the GBD initials on the stems. That insert added a further "touch of class" to the pipes and in London it was attached immediately.
http://www.thepiperack.com/blue-chip-unsmoked-pipe-1920s-gbd-xtra-french-hallmarked-billiard-w-box-and-sleeve/