"Olden days"??? Maybe for young-uns like you. For me, the olden days all those parts were brass.
well, they started using plastic in the '60 , which is fairly old.
My Super 20 (1970) has lacquered nickel silver on the thumb and lacquer is gone.
OP, it may have taken few months but the type of lacquer and its thickness may be varying these days, since all the talking about how lacquer would reduce vibrations (it doesn't).
By the way, I have seen vintage horns with a patch by the place where the tip of the thumb would rest, apparently some people have been able to wear through the metal (we spoke about it in the past too) let alone the lacquer.
Rampone & Cazzani had an outstanding solution fusing the virtues of plastic to metal
this was their Thumbrest in the '60 and '70 (still olden days, in my book) you see the extended plastic patch and the nickel plated metal hook ( worn out and wearing nickel plate is a tough job) , great protection.
Rampone & Cazzani, by he way makes metal thumb hooks coated with thick plastic dip these days