The rectangular horizontal piece that is mounted on top of the cylinder is not part of the original engine. The top fin may have had to have a flat machined into it to accomodate that block. I have no idea what function the round part that is screwed into that block fulfils. The engine appears to be rather small, looks like the 26 cc. The seller stated that the intended airplane outgrew this engine so my guess would be that it was intended for a military drone.