Well, we have an early winner! Manganese is correct.
"Manganese is used as a whitener and preventive of oxidation and it also tends to make the ingots less hard, and if carbon is present, to rid the melt of this deleterious element which, if present in sufficient quantity and annealing temperature is unduly high, will result in brittleness. The addition of manganese may be as high as 0.0075 percent without bad effects....With regard to manganese, this metal is added to each melt of ingots in the proportion of 1/20 of 1% of the weight of the melt."
"Methods of Coining Five Cent Copper-Nickel Coins in United States Mints at Philadelphia and San Francisco," December 19, 1938. [ RG104 Entry 328H Box 2.]