Panther variants designation is a highly controversial topic. There are many conflicting pieces of information, some sources are based on Allied intelligence reports and many info has just been invented postwar to make some sense of the whole Panther versioning mess.
First, you must appreciate that the Panther tank was constantly changing. There are several different variants inside each of 3 major versions (D, A, G). Also, all versions were used in parallel and it was not unusual that on repairs older models were fitted with newer parts creating even more confusing hybrids.
Second, as you probably know the Panther tank was intended to be used in Operation Citadel e.g. attack on the Kursk salient on the Eastern front. As Panther was in the prototype and early production stage at the time many things got rushed, some requirements couldn't be made at all, some couldn't be made on time for the Citadel, some were unified etc...
But the most common timeline of events goes something like this.
The very first version vas actually Ausf A.
This was the designation for so-called Null-serie of 20 prototype vehicles. They had weaker HL 210 engine, an earlier version of Kwk42 gun (single chamber muzzle brake) and maybe just 60 mm front hull armor (but some says it was already upgraded to 80 mm).
They were never used in combat.
Some designate this batch as A1 to differentiate them from later Ausf "A after D" versions.
Some say that this batch was later redesignated to D1 - but read on.
Production apparently started on Dec 1942 and this was tank intended to be used at Kursk. First 250 vehicles were fitted with HL 210 engine. It has 80 mm hull front armor, standard gun (double chamber muzzle brake).
This version got famous at Kursk for poor reliability.
It is usually designated as D1 to differentiate them from later versions with the HL230 engine.
Later Ausf D that some designates as D2 was fitted with final HL 230 engine and have many reliability improvements either on the production floor or in the field. Some designate as D2 only the variants with Schürzen side skirt upgrades.
Production of this model was ordered stopped on July 1943 but last ones rolled off on September 1943.
That was the first actual production version. Production started on August 1943 with many reliability changes, redesigned turret, MG mountings etc...
Some 2.200 vehicles were produced until May 1944 (or maybe even later)
Now, the exact reason why it was designated as "A" instead of logical E is not known.
But we can assume that this was intentional "political" decision and not some kind of error.
Possible reasons might be:
- This model finally incorporated all the requirements dating back to the autumn of 1942 without shortcuts being made because of the Kursk deadline
- A bad reputation of Ausf D might be the factor to discard the whole "Ausf D" batch as a kind of prototype/experimental vehicles and to start over.
Since the "Ausf E" is later skipped, my theory is that "Ausf A" was originally supposed to be "Ausf E".
Some designate this variant also as Ausf A2.
This variant was linked to the Panther II project. It was started around Jan-Mar 1943 when Hitler requested to increase hull armor to 100 mm and that couldn't be incorporated into the original Panther design. Panther II project was dragged on and quietly shut down.
It made several advances that wanted to be incorporated into the existing Panther design.
Most major of this was Schmalturm or small turret. Basic idea was to create smaller turret with a minimized frontal area that would reduce weight but keep the same level of protection.
It was never finished, and there were just a few prototypes but some other things from Panther II were later incorporated into Ausf G.
There were also experiments of Ausf F powered with gas turbine (GT-101 version of BMW003 jet engine), ideas to mount Jagdpather's 88mm Kwk43/L71 gun and so on.
As Panther II started in early 1943 and the earliest date for start of Ausf G project I could find was May 1943 (and this seems a little too early for me) the Ausf F was probably started somewhere during the spring of 1943.
The most common, most known and most produced version that started the production on March 1944. I won't write about it here as it is easily found all around the internet.