What this coin looks like (obverse, reverse, mint mark location, special features, etc.): One rare error of the 1943 D steel cent (penny) is the boldly doubled mintmark (RPM). The best way to identify this coin is to notice a very distinct and noticeable doubling of the "D" mint mark. A bold "D" is stamped on top of a very noticeable but more subtle "D" mintmark separated by great distance as depicted in the example image of the coin above. Source of example image is courtesy of Heritage Auctions.
Coin value chart: Typical coin prices, values and worth in USD based on grade/condition