End-Roll Toned 1964 Kennedy Half

Years ago I picked up a double roll (40 coins) of halves at a bank and it turned out to be all 1964 coins, in BU condition. I thought it must be an original roll except it contained 39 Philadelphia coins and a single Denver coin. So, it was probably put together by a collector and somehow ended up in the bank. Lucky me!

This coin was on the end of the roll, with the obverse side out. I thought it was interesting how it acquired the speckled grey toning on the exposed obverse only. The reverse had been protected from the air and remained fully brilliant.