Dale,
You may know Westinghouse material, (and I do not), but I do know Schon material. For you to tell me the Schon stuff does not turn yellow all the way through is wrong. I have my doubts that you actually have any 25 year old Schon material in your possession like I do. I just went and scraped deep into one of those ferrules and it was only slightly lighter in yellow color deeper in.
Would you care to bet your house on it?
The material I have came from Schon - who got it from Westinghouse.
Have a nice day.
Dale(2014 - 1976 = 38 years, but who's counting)