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

B or W: Reported to indicate the color of optional D88 hood and deck stripes.
L: Probably the most controversial is the letter L found on Malibu series Chevelles. It's strongly believed this letter L indicates external SS Equipment trim but this has NOT been confirmed by any direct knowledge or GM documentation but SS Equipment optioned Chevelles have been found without the L. While using this letter as a way to determine if your 1972 Chevelle is a true SS or not is up to you, there is no 100% guarantee but there is strong evidence that it does.
Z: Found on 13380 El Camino, 13437 Chevelle sport coupe, and 13469 4-door sedan. Believed to have body moldings meaning.

The 5-digit number following the body date is a concatenation of a 4-digit found in block #24 of the build sheet and a single digit that its origin is not known. The last digit of the 5-digit number after the body date is suspected to be the work day of the week. To date no number larger than 5 has been reported here.