Only #4 & #6 lacks the whitish cast typical of the glaucous undersurface of F. albicans, a white ash. In contrast, note the yellowish cast of the F. pennsylvanica surface. When I complained about not being able to see differences to Guy Nesom, our Fraxinus expert, he replied:
The difference between the two species is striking and should be immediately obvious ... The lower surface of F. albicans is the same as that of F. americana, with a dense overlay of white-reticulate wax. That of F. pennsylvanica has nothing obscuring the epidermal cells.
Thanks to Guy I now know what to look for, and I'm finding it — but it requires good magnification and lighting. With practice I may learn to see the difference in the field as well.
