Thoughts on this 1953 Proof Set?

I don't know. I collected original Mint and Proof sets for over 40 years, had them all except the early modern Proofs of the '30s. Searched through tens of thousands of them, maybe hundreds of thousands of them, and it was not at all uncommon for only 1 or 2 coins out of the complete set to tone. And in almost every case the nickel was the last to tone.

Of course there were others where every coin toned. It's really a matter of chance.

But those original sets were completely different than these after market sets. Something about that colored plastic, and it never mattered what color - even white.