Pattern letters were to enable quality control to identify damaged working patterns, or to identify which foundryman was using what pattern, either, again, for quality control, or for piecework pay. Since all working patterns were cast from an original master pattern, there would be no reason to pair up specific working patterns into sets.
You may notice there are Griswold waffle irons with pattern number 151N that you will always find on both paddles in a set. This is because those irons use the same pattern for both paddles, and the N stands for "New", rather than as a working pattern identifier. Letters for that purpose are found on the inside surface of the handle shank.