Bonded faceplate from Sylvania.
http://www.americanradiohistory.com/...-News-1960.pdf
This used a glass bonded faceplate, presume the one in the Pye is the same type. Sylvania did produce a smaller 16 inch CRT with a plastic faceplate for portable TV's.
I seem to remember the bonded faceplate CRT's by Mullard and Mazda had a plastic faceplate even in the larger screen sizes. Then the Fenbridge guard (cap) came along as noted by John in the GEC 2013 thread.