[F19Gladiator]

A theory or two only:
1.
It was not uncommon practice that a number of victories was painted within a wrath of for example oak leaves after receiving a decoration such as the RK, or perhaps after an even number of victories.
Can it be that the top of the rudder was to be decorated by such a decoration commemorating an award or achievement? The picture of this F-2 shows 18 victory markings, whereof the first two seem to have circles or cockades painted above, followed by the others having a dark "spot" visible above the remaining bars, possibly red stars.

Assuming that the commemorative victory emblem would have been painted as the "Half century" celebration after 50 victories and comparing with the victory listing of Oeasau at http://www.luftwaffe.cz/oesau.html
The following is possible:

After his 50:th victory, a Spitfire on 16.5.1941, he is listed for a further Hurricane on 28.5.1941 over Pas-de-Calais. Depending on the actual known confirmed and approved victories at the time, the first two bars with roundels might represent his last two kills assumed won in the West. (Plus or minus one victory is possible at the time, I believe.)
The remaining 16 with red stars are then consequently his first Barbarossa campaign victories

2.
However as no such painted commemoration is visible, another theory mentioned earlier on this thread is simpler and perhaps more plausible:
As his victories from No. 49 and onwards came while flying with JG 3, according to the same source above, the theory that the bars only represent Oesau's victories while flying for JG 3 is possible.

The first two with roundels on top would then represent two out of his last three confirmed victories on the Channel front. The third was perhaps confirmed later? The remaining 16 are consequently the first 16 victories from Barbarossa.
If so, this photo might have occurred around the first week of July 1941when he might have been in the Ukraine area. Vegetation and nature in the background does no speak against time and location, I believe.

According to the above it might well have been the 109 of Oesau. - At least far from impossible.

Just some thoughts..........