According to "Luftwaffe Codes, Markings and Units 1939-45", aircraft coded 6Z+?? could have belonged to any of these three units:
Schleppgruppe 1
examples listed:
6Z+?? - Bf110
6Z+SC - He111
More details from here -
http://www.ww2.dk/air/transport/schlep1.htm
Schleppgruppe 1 was known as
Transportgruppe Herzog from February 1945 until March 1945.
KGrzbV 600
example listed:
6Z+KX - Ju52/3m
The main operational code of KGrzbV 600 was C6+?? but the code 6Z+?? is listed to a Ju52 (W.Nr.7070 coded 6Z+KX) which was probably on loan or transferred from Transportgruppe Herzog
Hope this helps