Thanks for the history Demsey. My experience is similar.
Back onto the 6542, I have unearthed a couple of interesting tidbits about Perspex, the material that was selected for the original GMT's bezel insert.
Apparently, it was not by chance that Rolex settled upon the relatively fragile Perspex. In fact, the name Perspex has been ringing bells in the back of my head for months now, but I have been unable to make the connection. Until today. While I was going through some of my old British motorcar manuals doing some research for a friend, I found the answer.........Beginning in the 1940s & continuing well into the 1950s, (Standard) Triumph, Morris Garage (MG), Jaguar & other British sports car makers used a plastic material called Perspex to make the fixed & sliding windows that preceded roll-up glass windows in some of their cars (for some of you younguns, it was not that long ago when roll-up glass windows were an expensive option in sports cars)
More importantly, the same material, Perspex, because it was easily molded into many shapes & was somewhat anti-reflective, was also used to make windscreens & machine gun turrets in planes used in the 2nd world war
Of course, due to Pan Am's familiarity with airplanes (Pan Am was an airline, after all), Perspex was an obvious (& readily accessible off-the-shelf) choice for the GMT's insert.
So, now we know how & why the GMT got its Perspex insert