I think the Hawid mounts in 1945 were the first, at least according to their website:
Quote:
The invention of the protective mount and the beginnings of HAWID date back to the year 1945 in Berlin. The engineer and HAWID founder, Hans Widmaier, at that time a passionate collector of postage stamps, was looking for a secure protection for his comprehensive stamp collection. Hans Widmaier was familiar with polystyrene film from his time with Siemens. The film was used there for the protection of marine cable. He buries his stamps - enclosed in polystyrene film - in the ground. The stamps survive the time in their hideout entirely unharmed!
The idea for protecting stamps was born, and after initial obstacles and costly pioneering work, began its triumphal progress around the world as Hawid mount®.
http://www.hawid.de/hawid_engl./index_e.htmlOf course, for those in the US, it might be worth noting that Showgard places the date as January 1960 when the first Showgard mounts with the black background appeared in the US:
http://www.showgard.com/about_us.cfm