First off, your thinking is a little off. <<So I was just curious why some indian head pennies come out on the copper side and some come out on the zinc side>> there are two sides. A copper side, and a reject side. Not a zinc side. ANYTHING that is not the same as the copper goes to the reject side. That includes MOST wheats from 1942 back, plus 1943 steelies, Indian heads, and foriegn coins (Except canadian coppers, which go to the copper side since they are close enough to ours in composition).
Knowing that, here is what I do. First off, I do NOT run all the coins a second time. that wears the machine down faster, and for a mere one or two coins here and there. If I use that time running a new box instead, I find a lot more wheats from the new box, incuding old ones, rather than the one or two I might find in re-running the same box again by running the 80% that are rejects. The machine needs cleaned every 150,000 coins. Later it becomes every 100,000 coins. The parts need replaced down the road. I have six discs sitting here, all used up, for example. To do the work twice isn't worth it. Instead, WHILE I am running the Ryedale, I keep pouring my finds in the top, and watching the REJECT side with my eyes. Most rejects are shiny. Some are dark brown. Those I LOOK at, AS the machine is running. It is easy to do, fast, and you can flip the ones that land face up and see if they are a wheat or not. yes I DO get some older wheats in that reject side. Plus almost ALL the indian heads go to the reject side. So i watch there. I did some testing in the beginning, but saw that I got most of the wheat rejects as they came out. If I missed one coin every 10 boxes, even if it is a 1932 wheat (Arbitrary date, butone that would reject), it is not worth running the entire reject side again just for that one coin in ten boxes. If I run ten new boxes instead, I'll find that 1932 or equivilant in another box plus 90 more wheats of varying dates.
So don't run them twice. Use a 1981 as your comparitor coin. But keep your eyes peeled on the reject side and you'll find the ones that are keepers that go over there. remember - ONLY the older wheats and IHs go there. And they do NOT look like the 1996 that is brownish that goes there too. You'll see the differences pretty easily.
And your answer is yes, it is because the composition changed over the years. About 1944 the composition changed to the same as the 1981 you use as the comparitor. Some 1944 PHillies go to the reject side though, even though 1944 Denver goes to the copper side.
