Indeed [Artist Letter] is not an MC default field. It's a custom defined field. Your friend "Mark" may be able to copy the definition of his field in the "Calculated Data" section of that field in his library.
I looked in my library and found one that I apparently defined a very long time ago. I don't even remember doing it. Mine is called [Artist_Letters]. The expression in my field is:
Regex([Artist], /#^(?:(?:the|an|a) +)?(.)#/,-1)if(Compare(1[R1], >=, 10), 0 - 9, [R1])
I'm pretty sure I wrote that because I tend to use Regex for a lot of things. My definition strips out "the", "an", or "a" from the beginning of an artist name. You could add more there if you have artists that start with some other article like "La", "Le", "Los", etc.
You might also notice that I'm accounting for Artists that start with a number. In those cases I'm outputting "0 - 9" as the "letter" of the artist. Thinking about it more, it might make more sense to output a hash symbol (#) instead. In fact, it might make more sense to output "#" for anything that's not a letter. Here's an expression that does that instead:
Regex([Artist], /#^(?:(?:the|an|a) +)?(.)#/,-1)if(IsRange([R1], a-z),[R1],#)
Brian.