Query: For what part of a motion picture projector did the Motion Picture Trust hold a patent?
Context: For Laemmle and other such entrepreneurs, the creation in 1908 of the Edison-backed Motion Picture Trust meant that exhibitors were expected to pay fees for Trust-produced films they showed. Based on the Latham Loop used in cameras and projectors, along with other patents, the Trust collected fees on all aspects of movie production and exhibition, and attempted to enforce a monopoly on distribution.