Originally Posted by Feuerfalke

During the Battle of Britain only the rear plate was replaced as additional protection. The square canopy was added later. As such, you would probably not even recognize the difference unless you have very detailed shots of the different 109s. If it was even worth modeling at all...

Besides that, when the square canopys were added to the E4s, the E3s also received them as a refit. So even that wouldn't allow a 100% identification.

@ Sven:
Correct. Given the little differences and the fact that the gun is already modeled makes me quite confident it's either in the release as well, or will be added in a patch.