The 13 YB-17s were individually/hand made so many variations and equipment changes. Guessing two differing mounts trialed, possible shown on two different aircraft. In my attempts to pin down Bell XFM-1/YFM-1 photos and data for model building, I found that the 13 service trial aircraft varied upon completion as well as continuous modifications during their life. I have yet to find enough photos of one serial number at the same time to allow an accurate build. In fact, that's what led me to this forum in search of photos. From the photos I have seen of the YB-17s/Y1B-17s and later B-17Bs they can be just as ambiguous.