Help Identifying What Happened To These Nickels.

It could be environmental damage. I have dug many US nickels that are darkened and look just like yours.

The surface texture is wrong for a Henning and also Hennings usually do not have full sharp rims and details. Sorry but I don't see any Hennings here. Also as far as the extra date, I think that comes from Henning claiming to authoritities he made 6 obverse dies and only told them about 5 dates. So people assume he made another date that he didn't disclose. My theory is there were 2 dies dated 1944 since this was the first year he counterfeited and after a bank teller commented how interesting it was that all of the nickels he was cashing in were 1944's he decided to make dies with other dates to mix them up. I think that is why the 1944 Hennings are the most common. This is just my opinion.

Also in the group above the 1954 would be the only possibility since Henning was arrested around 1954/1955. So he wouldn't have had a 1957 or a 1964 nickel to use for a host coin to make his die. Study the characteristics of a Henning on the other threads and keep looking. I personally just check coins with the known Henning dates but that could just be too much confidence in my theory. Good luck and make sure to continue posting what you find.