Excuse me for breaking into this, but perhaps I can help. You asked what unit 2./schw.Flak-Abt 306(o) (2nd Battery of Heavy Antiaircraft Detachment/Battalion 306 (stationary)) belonged to. As you can see from this link,
http://www.ww2.dk/ground/flak/abt/s306.html , it's impossible to answer that question without an exact date, and extremely difficult to answer for dates prior to about mid-1943. The Luftwaffe's Flakkarten (Flak organizational and deployment charts) begin with the chart for 1.11.43 and end with the chart for 1.12.44. The ones before and those after these dates did not survive the war. Moreover, the Luftwaffe's Flak batteries moved around a lot: sort of "here today, gone tomorrow". However, this particular Abteilung (306) was stationary and did not move around much, so it was probably deployed around Flensburg from 1941 until it moved to Lübeck in June 1944. As the link shows, a Flak-Abt. was typically subordinated to an ever changed number of Flak-Regimenter, which in turn were subordinated to a Flak-Division.
That's about all that can be squeezed out of this without additional information.
L.