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.  These seem to begin appearing in February.
L: Probably the most controversial is the letter L found on Malibu series Chevelles. It's strongly believed this letter L indicates external SS396 or SS454 trim but this has NOT been confirmed by any direct knowledge or GM documentation. One known SS396 optioned Malibu has been reported (but not verified) without this letter L.  While using this letter as a way to determine if your 1970 Chevelle is a true SS396 or SS454 or not is up to you, there is no 100% guarantee but there is strong evidence that it does.
X: Found on 13369 Chevelle 4-Door Sedan.
Z: Found on 13437 Chevelle sport coupe and 13480 El Camino.

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 known.