Well as you should know the 40% halves are similar to the regular clad in the sense that they are clad/plated(to my knowledge), just 1965-70 are silver clad. It looks like some of the silver clad layer has come off. I am not entirely sure what causes that but my guess would be either a flaw in the metal or a flaw in the die. Perhaps when the dies struck the coin it wasn't done perfectly and some of the silver clad layer of the coin stuck to the die when the die was removed. Thats my best guess.
I have a 1943 Canadian tombac nickel with a similar issue. There is a bit of a chunk taken out of my nickel but the design is lightly still stamped into the coin where the small chunk is missing. Then the dip in the coins hits a part of the coin where (it is similar to the coin pictured here) the coin has a thin layer of metal that is sort of curling up. I will attempt to post a picture some time.
I would take it to a coin dealer and get an expert opinion on it.