I'm not sure about the SF code. It seems Schleppgruppe 1 did not operate with this aircraft. However, toward the end of the war, whatever was available may have been used. The codes used were F7+ and 6Z+.
https://www.asisbiz.com/Luftwaffe/schlep1.html
From another forum: "There is a complete and very detailed history of Schleppgruppe 1/Gruppe "Herzog" in an article by Georg Schlaug published in
Jet & Prop magazin, Heft 2/2000, pages 57-65. It covers all of its operations to Posen, Glogau and Breslau, January to April 1945."
Ed