The Bicentennial 3-pc. Silver Proof Set has three 40% Silver Proof coins:
Washington quarter, JFK Half-Dollar and
Eisenhower dollar,
dated "1776-1976" and with Bicentennial reverse designs.
1992 is the first year where the 'standard' Proof Sets (1˘ to 50˘ denomination set) were offered with the choice of Clad (copper-nickel)
or (90%) Silver, referring to the 10˘-to-50˘ coins in the sets.
The 1970 Proof Set has a 40% Silver Half-Dollar, the rest of the (10˘-to-50˘) coins in Proof Sets from 1970-91 are Clad (except for the
Bicentennial 3-pc. Silver Proof Set listed above).