Constant OLD_MUSIC21_NAMES 

pub const OLD_MUSIC21_NAMES: [(&str, Option<&str>, Option<&str>); 21];

The unique names music21 kept an older spelling of: its oldMusic21Abbrev and oldMusic21WorkId, both of which Metadata::search takes as a field name. Each row is the unique name, its Humdrum abbreviation and the work id it was written as before music21 v8.