What would cause this?A BB?

[kbmuri]

There's a soft plastic glare filter between the front glass and the CRT. It's melted and wrinkled, and the very center is stuck to the front glass. It was originally gray, not yellow/brown. I'm sure it would scrape off. Looks like a good CRT behind it. If it had been stored in a vacuum from day 1, that filter would still probably have decomposed, so it's probably technically correct to say it's "mint for its age". Rest of the set looks mint for any age.