Originally Posted by
xliioper
Yes, good info. As noted, the 'C' in second to last letter indicates the fare books into W class. If second to last letter is an 'I', the fare books into the C+ S booking class. The S class version of fares is $150 cheaper on a each-way basis than W fares (the Q fares are shown below, but differential is same for the L fares). If there was S bucket open in one direction, total price would be $150 cheaper and price would be $300 cheaper if S bucket was available in both directions.

Thanks to both xliioper and findark !!
Does this mean that Delta may not yet have opened up some better (cheaper) fare codes? I've seen that referenced before, that bookings far in advance don't have all the normal fare codes available.