Quite a few overweight (3.3 - 3.4 grams), "brass" cents (cents enriched in tin and/or zinc) appeared in 1941 and a few in 1942. Their specifications don't correspond to any foreign planchets being struck by the US Mint at the time and it's unlikely they represent cents struck on foreign stock. The most likely, and most prosaic explanation is that they were punched out of strip that was was both enriched in tin and zinc (improper alloy mix errors) and rolled too thick.