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

B: B has been found on El Camino/GMC Sprint and 13857 Monte Carlo and sometimes with D and does NOT indicate SS Equipment.
D: D has been found on 13637 sport coupes, convertibles and Custom El Caminos and whether or not this indicates a Z15 SS Equipment optioned or D88 Hood & Deck Stripe optioned Chevelle is not confirmed.