I have piano sheet music (~100 measures). At two bars, there's a letter in parentheses added - (a) and (e). Due to the context, I don't think it's chord symbols.
My first guess was, it's additional notes that should/could(?) be played here (similar to the first example in this question). I wouldn't know, whether to play the note in addition or in exchange to one of the other notes, though.
On the other hand, I would have expected the notehead in parentheses, not its name. Hence, I wonder if these letters denote something else. Does anyone know?
Background info: This is from a piano arrangement of "I'd do anything for love" (M/T Jim Steinmann). It seems, the score is from the magazine "Keyboards", probably the issue 1/1994. The score itself is not available via the linked online archive. The arranger Wolfgang Wierzyk has published several score collections and own compositions - but I couldn't find this song so far.