Today in Postal History

This airmail cover went into the mail in Tiranë, the capital of Albania.
Here's the hometown newspaper.

The routing requested airmail via Salonica (Thessaloniki) to the destination of Athens.
Thessaloniki was east of Tiranë on the Aegean Sea where it arrived June 5.
It arrived in Athens June 7.

There are two transit marks for these stops on the back.
There is also a faint airmail handstamp showing a biplane on the back.
There is also a circle with a 36 which puzzles me.Can someone explain this?
The other faint circle on the back is probably an offset from another cover in the stack.

The franking consists of a 15q. dark brown King Zog issued in 1930,
and two airmails, 5q. yellow green and 15 q. rose red showing
King Zog and an airplane over Tiranë (Scott 252, C29, and C30).