Previous to the "D" block designation it would have been the "A" for 1960 and some 1961 models, the "D" was introduced in the 1961 model year as well and the main difference was it was a 2bbl version rather than 1bbl and ended up replacing the "A" version all together by 1962. Like Ed Snyder said The "C" version was a heavy duty version used in bigger trucks, it had the longest run of all the GMC V6 engines for 15 consecutive model years up through 1974.
