Hi Steve. I would also say that the primary section of the map would be the group of letters and numbers I would use. Of course it has been a long time since I have studied my maps but that is what I vaguely remember when I was heavily involved accumulating my large collection of maps. Since each map may have other smaller sections inserted there is no reason to think they updated each smaller section upon insertion especially if there were no significant changes. So there would be slight variations as to the dates in the collective grouping of inserts when compared to the primary map.