>>3255768It is 100% possible to reverse the yellowing on the rear glass on that specific lens.
Got one from my uncle as a gift and it was yellowed beyond use. I did some research and found out that UV light can reverse the yellowing on thorium coated lenses. I left the lens in a window for a few months and when I came back it was completely reversed.
Pic is my lens. The yellowing has yet to return.