In Die deutschen Flugzeuge 1933-1945 page 946, there is a photo of a Do-217 with a very similar payload. Caption reads "als Versuchstrager fur Saenger- Staustranlrohr". And of course I can't read German. Looks like a large long pulse jet.
NJACO,.. also noticed the stabilising fins on rear fuselage ventral area.
Can someone interpret this and the German on the photo above, left hand side?.