Well note the die state. The proof coins are only used to strike 3,000 coins. (total of 6,000 strikes per side) Thus the surfaces show little die wear. (In face most proof coins show even more die wear than this one) Thus one was probably cloudy on the fields and was spent because of the bad look to it. It could be added to a collection, but you could get better examples in BU than this coin. People save them, but they are just an impaired coin. If it were a variety I would save it, but if it were proof looking, it could be spent. (Many times these coins were left in their original cello and turn strange colors) Or it may have been put into s soft flip and had been stored for a time leaving PVC damage on the coin, and then discarded to circulation. Seeing the crispness of inside rim makes me think it is a retired proof coin.

Sometimes you can see rim scratches on the edges of proof coins:
