Thanks Seth! I haven't had the time yet to run through different references (haveing only just procured a few myself...). I'll need to look up his reasoning for the difference when I get a chance...
Not sure about your attribution. The reverse MIGHT be the Dolphin from a scene of Beetlejuice I The Great:
You will very likely not find a comprehensive enough explanation from Poey d'Avant but the most likely reason for the difference is that although the main type is carried from a lord to another (by immobilisé), monograms tend to get stylized as the blueprint type gets older and older. You see this mostly in the later Carolingian and Robertine coinage but also later in the 12-13th centuries.
I can understand the evolution of a monogram, but I'm curious about the reasoning for why one monogram design fits to one ruler, and then a particular evolution is attributed to another. That would seem too convenient for us collectors who like nice little boxes and checklists. I'm more than happy to accept it if there is a good reason though!
I also think he mostly applied this logic of artistic interpretation of old design devices on the immobilisé types.
Which is that the more a monogram on the subsequent type differs from the blueprint issue, the later it must be. Adding to the clues Henri's name clearly visible and the overall appearance of the engraving, the weight of the coin and the style, you likely have Henri II. I agree it is mostly conjecture and convention.