I just started about 6 months ago sorting nickels because my ryedales sort my pennies for me. I have been pulling all pre 1963 Nickels. I came across exactly 40 (one roll) BU like 1961 (plain) nickels in a bag the other day. These nickels stick out like a sore thumb compared to more circulated nickels. They have a kind of gold ting to them and the face is so pronounced. Pretty Cool to finally get some old BU coins.
I don't understand the BU abbreviation used to describe coins found because you found them in circulation, but these nicks are the closest thing to BU I've seen sorting nickels.