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. While using this letter as a way to determine if your 1971 Chevelle is a true SS or not is up to you, there is no 100% guarantee but there is strong evidence that it does.
X: Found on 13236 Nomad station wagon
Z: Found on 13236 Nomad station wagon, 13380 El Camino, and 13437 Chevelle sport coupe.

The 5-digit number after the body date is a concatenation of two other numbers. The first 4 digits are the 4-digit data processing number found in block #24 and the last digit is suspected to be (but not confirmed) the word day number of the week; 1 would be Monday, 2 is Tuesday, etc.