could be quick change version. Ie by night a freighter and by day a passenger aircraft. seats come on pallets. Popular on the 737 and BAe146 as well as many smaller turboprops such as ATR, Metro , C406. Otherwise a combi, where some of the cabin is used for freight and some for passengers. Ie 747-300 varients.
As Taiko and intruder say its easier and cheaper to do as little as possible on many conversion progs. so often the windows are left in.