To me, the trailer with the straight drawbar seems to be the same, as the one shown in the attached photo,
which is described by Ernst Peter in "Der Flugzeugschlepp von den Anfängen bis heute" (aerotow from its beginning
until today), Motorbuchverlag 1981.
It's designated as the Fieseler P 21A (so no Siebel project or design) and was just the start of the development of a
flying trailer, which could be used as an auxiliary tank, or as a bomb carrier, or as both, as shown in the second picture
from the same source.
The version on the second photo above perhaps just differs with regards to the drawbar, which may give
the vehicle a more streamlined shape, I think. Interesting sidenote is, that after encountering problems with
stability at low speeds, a pilots seat was added to the trailer. Due to the small size of the trailer, this was
only suitable for small pilots, so Hanna Reitsch was committed as test pilot.
There are also photos of a He 177 towing a trailer with a tank above the wing. Those tests actually weren't
aimed at increasing range, but for testing the behaviour of drop tanks being hit by gun fire. The towed tank
was fired at by the rear gunner of the He 177 to get data, that should help with the development of conventionally
carried tanks for fighter aircraft.