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

B: B has been found on Malibu sport coupe, sport sedan, convertible, 4-door sedan, Malibu series Custom El Camino, and L6 Greenbrier station wagon, sometimes with D and sometimes by itself and does NOT indicate SS396 or SS454 but does not preclude the car being an SS396 or SS454 either.
D: D has been found on Malibu sport coupe, convertible, and Malibu series Custom El Camino, so far always in conjunction with B.
L: L has been found on Custom El Caminos.