You are almost certainly correct. SWarp was used to co-add a number of subs. As the intention was to acquire an image, any image, no biases, darks, flats, bad pixel masks or anything else were used. The filter wheel was playing up and I can no longer remember what, if any, filter was used. The FITS headers claim no filter, which seems plausible, but I wouldn’t swear to it.
British Astronomical Association
Supporting amateur astronomers since 1890