GL is the official abbreviation of “Generalluftzeugmeister”, at the time Erhard Milch (after Ernst Udet had committed suicide on 10th Nov 1941, shooting himself with his S&W M1917 .45 revolver) had this position.
Its exact organizational structure is not known to me, but obviously (to someone who worked in the german civil service), the distribution list says:
GL/C is one of its directorates, I believe C is “Technisches Amt” (could be Planungsamt).
E 6 is a section of directorate C and E 6 V is a subsection of E 6 responsible for the ammunition.
“GL/C - Chef” is the Chief of Staff of directorate C (only to inform him on the existence of the report, he will not want to be bothered with details and therefore gets only the cover page)
K.d.E. - in my opinion “Kommando der Erprobungsstellen” the military body in charge of all Luftwaffe proving grounds regarding adminstrative, disciplinary matters and the like.
“GL/C-E Chef” goes to the Chief of Staff of GL/C-E who will pass it on to the head of GL/C-E6 (satisfying hierarchically correct red tape).
GL/C-E 6 V - at last the copies for subsection V of section E 6 where the actual work on the problem treated in this report is done.
E[rprobungs]stelle Tarnewitz (as you know) also worked on armament matters (FG 42 was tested there)
J and E6 are in my view addresses within the originating Erprobungsstelle Rechlin. I would expect J to be the library and E6 the originating group (same as unreadable bottom left corner).
By the way, Dr. Burgsmüller later became head of the Bundeswehr Erprobungsstelle 91 at Meppen.
you have assembled a very impressive paper, which will take some time to take in.