[Dénes Bernád]

After checking THE source on Bücker airplanes, the book by Erwin König, printed by NARA-Verlag in Switzerland, I can give myself a (partial) answer.

There was indeed a version of the Student with enclosed cockpit.
In the book, only photos of D-EBRQ and -ESBS are given. However, my photo shows the first two letters of the civilian code as D-ER_ _. The last letter might be a T, an E or an F.

Does anyone know the full code?

Thanks, again.