HP-46 color consciousness?

[Accutron]

This is just a guess, but it's possible they switched to a plastic that included brominated flame retardant. The two different plastic formulations would've appeared similar or identical at the time of manufacture, but of course the brominated plastic would yellow more with age. I have Commodore calculators which were made during the transition (right around 1973), which have a mix of brominated and non-brominated keys on the same machine. Even though all of the keys are apparently the same age, exposed to the same amount of sunlight etc., the non-brominated keys are still bright white while the brominated keys are evenly yellowed.