The E-7 is essentially a standardized version of the Emil which took all the modifications of the previous versions (most importantly those of the E-4) into the production process. The main difference was the installation of the necessary plumbing for a droptank (which the Emil could not use prior to the E-7) but apparently the droptanks weren't used before early 1941 (and therefor they're not relevant for the BoB).
Structurally neither the E-1 nor the E-3 nor the E-4 (or the E-7 for that matter) were different in any way. The difference which led to a different subseries was the wing armament - the E-1 used MG 17, the E-3 used MG FF and the E-4 used MG FF/M and in many many cases both earlier versions were reequipped with MG FF/M and therefor became E-4s. Any other details (such as the early and late type canopy and the armored headrest) cannot be used to discern the actual subversion.
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