What's at the core is irrelevant...Iagree with you there, Beaun.
At any rate, the OP has his answers. Could be interesting to explore this a little further though...maybe see what they think over in the chemistry forum. If for no other reason than to give myself a better understanding of the processes involved in copper contamination.
Wikipedia had a litle more detail...
1793"“1857 100% copper
1857"“1864 88% copper, 12% nickel (also known as NS-12)
1864"“1942 1946"“1962 bronze (95% copper, 5% tin and zinc)
1943 zinc-coated steel (also known as steel penny)
1944"“1946 brass (95% copper, 5% zinc)
1974 Experimental aluminum variety
1962"“1982 brass (95% copper, 5% zinc)
1982"“present* 97.5% zinc core, 2.5% copper plating