Without looking at the NGK book or website Steve I can't remember what the differences are with the second letter for the caps. It might just designate a difference in the angle at the elbow. What I can remember is the suffix letter which relates to the top electrode fitting of the cap. F designates the larger electrode, E designates the threaded top. When fitting aftermarket caps it is sometimes necessary to change the plugs to fit the caps. Over here for instance you cannot get XD05E caps which means you have to swap to a plug with a different electrode fitting.
Essentially though as long as the plug has the right reach, thread and heat range it really doesn't matter if it has one electrode or two. Consult the book, match up the plug and cap, install, ride.