Interesting 1951M penny with a good sized planchet flaw right on the rim. Probably due to a gas or carbon inclusion in the bronze. When the coin was put through the upsetting mill to apply a rim the the flaw was present giving rise to a low rim opposite the flaw. This is known as the “Blakesley Effect” and is typical of clipped planchet errors. Certainly interesting to see it on a coin with this sort of rim flaw.