[olefebvre]

As i underlined the only on paper difference is the addition of droptank support, just like the E-4 introduced MG-FFM to replace the MG-FF. In both case the a/c were upgraded to the new standard, but did not necessarily beneficiate from the other enhancements introduced on the production line which do not constitute a type by themselves just the evolution of the generic Emil type.
If they hadn't introduced the E-7 or E-4 references which were useful on a tactical standpoint you could have had E-3s with MG-FF and rounded canopy and E-3 with pointed spinner, square canopy and MG-FFM. The change in designation was dictated by operational consideration not by real change in design which made teh a/c so much different from the previous version.

So in my mind an E-7 is just an Emil with cannons and droptank support, and that was the point in introducing that designation, it does not cover anything else. Just like E-4 does not cover squared canopy, it happens that they were introduced at about hte same time as the MG-FFM in hte production line but they are no part of the official designation and early E-4 had the old style canopy.

Change in subversion designation on the 109 were dictated by tactical use, armament or engine, a change in one of those would incur a change in designation. An evolution of the basic design, for aerodynamical purposes for instance would not and they should not be relied upon to determinate a version as such changes spanned over several versions being introduced in the production process whenever possible.

Cheers,
Olivier