Early, mid, and late, are now divided by the planeset allowed in each. Late allows all, mid only up to a certain date, and early only the "early" ones...
Right around when the 109s came out some time (2.6 I believe?) there was a major revision to the airflow code as it pertains to wings and aircraft in general. Most aircraft were hit in some way (good or bad) and many fly totally different now. For some craft that this hit, it's much more realistic (no more floppy 109s! no more 250mph snap-stalls in 190s!) but has had different effects on others (p51s no longer are turning monsters, but f4us are uber beyond words)