According to Griehl/Dressell "He 177/277/274" P112/113 the device towed by the He 177 in
http://cgi.ebay.de/ws/eBayISAPI.dll...item=6205970796 was a roll-stabilised Schleppgerat designed to carry a 66 gal (300-ltr) auxiliary fuel tank.
A photo of TM+IU (Wk Nr 550229), probably from the same series, is to be found on page 113. This aircraft appears to be one of two allocated to DFS and was flight tested with the Schleppgerat during June 1944. A drawing of an A-5 and Schleppgerat is also to be found on page 112.