[JG14_Jagr]

Its important to understand what happens when a plane goes through a revision. Some things are modded in the field by the ground crews and the aircraft is brought up to the next level standard. Some changes require more significant work that can be done effectively in the field while still doing the maintenance needed to keep things flying. Often those planes would be rotated back while either new production AC or already retrofitted aircraft would be issued. If an airframe was still useful it would be upgraded until lost or damaged beyond repair. Many an Fw190A5 ended its life as an Fw190A8 after several upgrade cycles.

For an aircraft to be an Me109E3 and still exist today, it is likely it was lost or captured at a time that its upgrade cycle was frozen. It is unlikely that an E4 canopy would be put on an E3 since the parts would no longer be produced and the armored glass of later types was something the pilots wanted...same goes for the pilot armor. There are pictures online of AC that were built as E3's but they now appear to be E4's based on the added upgrades.. so technically you could call them E3's upgraded to E4 standards..