Various letters are found on the third line of the trim tag. These include:

B:  B has been found on 300 Deluxe and Malibu series, sometimes with M and sometimes with L, and sometimes by itself.
F:  F has been found on the El Camino body style often found with B and either M or L.
M:  M is most often found with B but has been found by itself on convertibles.
L:  Probably the most controversial letter. It's strongly believed this letter L indicates external SS396 trim but this has NOT been confirmed by any direct knowledge or GM documentation. The letter L has been found on at least 1 non-SS396 optioned Malibu and at least 1 known SS396 optioned Malibu has been found without this letter L.  While using this letter as a way to determine if your 1969 Chevelle is a true SS396 or not is up to you, there is no 100% guarantee but there is strong evidence that it does.