Ooops..
Yeah I guess the perception is because just about any BoB book goes into great details comparing the advantages and disadvantages of the 'Emil's cannon' and the 'RAFs eight gun battery'. No mention anywhere of all MG Emils. Gee, I am a revisionist again, THE BOOKS say it goddamnit!
Seriously, the thing is just that: if they changed
a spring in the MG FF cannon on an E-3, it became an MG FF/M, and that Emil become an E-4 because it had an ff/m.. the rest of the details were all shared accross the model line.
Changes in guns, engine and radio made a 'new' model, not other changes introduced in the midtime in production.. and these didn't have to be radical. The G-4 for example was just a G-2 with VHF radio, and the G-6 was just a G-4 with 13mm MGs... hell the 109G itself was just a 109F-4 with a new engine! In fact the spare part list of the Gustav only lists new parts, and otherwise refers back the to F parts list..