L = Laon Gradual, Codex Laudunensis 239, ca. 880; some sources inexplicably date it fifty years later. See here for notes concerning the value of this manuscript.
C = St. Gall Cantatorium, Codex Sangallensis 359, end of 9th cent.; some sources date it to 922–926; contains only graduals, alleluias, and tracts
E = Einsiedeln Gradual, Codex Einsidlensis 121, 960–996
Bam = Bamberg 6, 966-1000
G 339 = St. Gall Gradual & Sacramentary, Codex Sangallensis 339, 980–1000
MR = Mont Renaud Gradual, 10th cent.
Ch = Chartres 47, 10th cent.; destroyed in 1944
Non = Nonantola Antiphonale Missarum, 10th cent.; fragments
H = Hartker Antiphoner, Codex Sangallensis 390/391, 986–1011; for the Divine Office, not the Mass
Ang = Angelica 123, 10th or early 11th cent.
Bv 33 = Benevento 33, 10th or early 11th cent.
Mp = Antiphonary or Tonary of St. Benigne, Montpellier H 159, 11th cent.; digraphic alphabetical notation
Bv 34 = Benevento 34, 11th or 12th cent.; diastematic
omnigreg.at is invaluable for comparative analysis; use the search (Suche) bar to find a particular chant.
See also The Most Important Chant Manuscripts.