After a lengthy search I finally purchased my first GBD pipe, an estate silver-banded Apple which (according to the Seller) dates via the hallmarks to 1937. I only hope that I look as well when I am 77-years-old!
Meanwhile the GBD name was well established and thus retained. August Marechal and Ferdinand Ruchon led the firm into the 20th century. They were in charge of the company for more than 50 years.
"M R & C" stands for Marechal, Rouchon and Company, one of the various manufacturers of GBD or controlling entities of the GBD marque in the early to mid 20th century.