Hi Guys!

Here are my updates:

The seat post has a diameter of 31.7mm, while the seat pillar has 27.2mm. The top tube has a diameter of 28.6mm.

I also got an interesting response from Mr. Brumberg, the owner of MESU. He took over the company from his parents in 1972 but does not remember the logo. He knows that they changed it at some time, but has to look into their archive when he's back from holidays.

Furthermore, he told me that they sold exactly the posts in diameters my bike has, i.e. 28.6 and 31.7 to nearly all frame builders in Germany and Netherlands at that time. Their most prominent customer he remembered was Enik (de.wikipedia.org/wiki/Enik_(Fahrradhersteller)), who supported the Nordrhein-westphalia Racing Team. From old newspaper articles I know that Enik started building racing bikes around 1978 - a time that would not fit to the MESU decal, which is presumably older than 1970 (according to Mr. Brumberg).
Unfortunately, Enik went bankrupt in 2007 and got sold out. In 2016 the trade mark Enik was bought again.

I then looked for catalogues of Enik and found one (fotos.rennrad-news.de/s/14540). The scans are from 1983 to 1986 and show only two models with a frame made from "precision special light tubes", all others are Reynold 531 or Columbus tubes (models are Stelvio and Amateur 2S (before 1984)).

All in all, thats quite a lot to look up and research. I will definitely try to get in touch with someone from (former) Enik.

Let me know what you think or if you have further suggestions!