[olefebvre]

In fact there were a lot upgrades which were introduced in the 109 life which were designed to be retrofitted to earlier batches of produced a/c. While some introduced a change in designation like the equipment for droptanks some did not, the canopy for instance. There were hundreds of such modifications which should have been applied whenever possible.

Conversion to E-7 standard was indeed a must as it provided the Emil with a much needed extra endurance, and it was a definite high priority upgrade.
The WkNr 704 is typical of E-3s which went through the main upgrades, as it appears at a point as an E-4 and then as an E-7.

AFAIK only newly produced E-7 got the squared canopy and those earlier a/c which had them fitted at some point. The earlier a/c kept their canopy or they may have got a new squared one if time/spares where available or repairs needed.