Per the latest Coin World; I don't know that much about composition variances, but it sounds to me like they are saying something is amiss here.
that is the 1944 to 1946 composition. the 1942 is .95 copper; .050 tin & zinc. no silver - ever, except in trace amounts, possibly if there is silver, somethiing is very strange
FWIW, A principal (THE principal?) source of silver is copper mining. I can easily believe there was silver in someone's copper. I am not sure how much this complicates any answer. from Wiki
I think enough experts have looked at it to tell it was struck at the US Mint. The burning question is how a planchet like that (with silver) found it's way into the hopper. Equally disturbing, as I recall, they said there was no foreign planchets at the mint with that composition.
Rlm is correct on copper, zinc , and silver being commonly found together. Due to the scarcity of copper for the war effort and procurement foremost, maybe the refining accuracy was slack. However, I do not know of anyone having done a study ( say with the handheld x-ray diffraction devices) of the actual composition from the start of bronze small cents. I will add that to my list so if I can talk the local college into buying one, I can answer. I think the composition is similar to the official weight , with a tolerance. Jim
There was a pretty interesting discussion over on the NGC forum about the coin. RWB (Roger Burdette) started it (he was one of the numismatists quoted in the Stacks auction listing), no definitive answers on the coin though. http://boards.collectors-society.com/ubbthreads.php?ubb=showflat&Number=6264553#Post6264553 http://stacksbowers.com/news/article.aspx?id=239
According to Lange, all Lincolns prior to 1943 also contained tin. Although the exact percentage is not specified, I believe I have typically seen 1%. So that would be 95 copper, 4 zinc and 1 tin. i.e., the zinc is nearly double the specified quantity. Lincolns 1944 to 1946 contained no tin, but were to contain 5% zinc. From either if those specifications, 7.5% zinc seems a bit large to be considered "normal". Another complication is that there were numerous experimental cent struck in 1942 trying to find a substitute for the copper - both metallic and plastic.. Who know if this was not merely a test piece?
That was why they were talking with Roger Burdette, he has a book coming out shortly on the experimental cents of WWII dealing with the designs, alloys etc. Research having been done through examinations of the coins and the original source documents in the National Archives.
