Landon Ricketts mentions that he helped build the bridge when the player is standing near him.
Mason's Bridge and Dixon Crossing are probably a reference to the "Mason-Dixon Line", which marks the border between the Northern and Southern states in the U.S.
When walking across Mason's Bridge, Marston will occasionally state "Something isn't right here. This has also been known to occur while exploring other locations, such as Rancho Polvo, Scratching Post, or Aurora Basin.
Mason's Bridge will occasionally not appear on the map even after all the maps of West Elizabeth have been purachased.