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

X & Z:  Sometimes just the X, sometimes just the Z, sometimes together as XZ which are found on 131/132xx Nomad series and 133/134xx 300 Deluxe series.
T & W:  Found on a Concours optioned Malibu sport sedan.
L:  Probably the most controversial is the letter L found on 300 Deluxe (along with X) and Malibu series Chevelles. 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.

The 5-digit number following the body date is purported to decode to the xxxth car on the xxx work day of the week such as 05361 would decode to the 536th car on the 1st work day of the week. Whether this is true or not isn't verified.

AVO xxx: Purported to mean "Avoid Verbal Orders" although that seems rather odd. The number following the "AVO" text is sequential but not limited to Chevelles; Buick, Oldsmobile, and Pontiac A-body cars were being built at the Leeds plant in Kansas City and has been found on Buick tags as well.