This year saw the Mint supposedly test different alloys of nickel & copper using a single obverse and 4 reverse dies as shown below courtesy of PCGS J1704/P1908, J1707/P1911,
J1710/P1914 and J1712/P1916.
Other than the pure nickel coin which is magnetic, it is not known if the other alloys described on the reverses were actually used when striking the coins. It is likely that they were struck using standard nickel alloy which is 25% nickel and 75% Copper.
About 2 to 3 dozen where struck in nickel and about a half dozen were struck in alumimum. Although these are popularly collected as a set, it is unclear if the Mint deliberate produced these are sets. There are no original sets known to exist today. Dr Judd's set was sold in B/R 9/75 sale but may have assembled.