This is a 1943-S steel cent off-metal struck on a Mercury dime 90% silver planchet. The coin is silver, and weighs 2.5 grams, which is correct for a dime. A very popular year for errors since it is the year the steel cent was being struck, and also because it's a WWII year. There are off-metal cent on dimes for 1943 and 1943-S (there is only one 1943-D), but of the two, 1943-S cent on dimes are about 4-5X rarer than 1943-P cent on dimes.
NGC AU-53 with white surfaces and no problems. An above average specimen.