This is a section of a 1927 map that is the earliest one I have located that shows any reference to Preston or Preston Falls. The earliest USGS Topo Map that carries this name, the Cherry Lake, South 7.5' Quadrangle, was created in 1990. The larger 15' map of the same area, Lake Eleanor, does not show the feature because it was too small.
It seemed only a simple step from this point to contact the organizations producing the maps and ask to review their cartographers notes. This would explain the justification for adding place names to the maps. This was easy enough in the case of the USGS maps. Dale Benson, of the USGS Cartographic Division, Denver, CO sent me a copy of the Field Notes relating to Preston Falls. Fortunately all of the people used as references for the name were still alive and known to me personally. I spoke to each of them and all gave the same reason: "common usage". That is to say, they didn't know the origin of the name but everyone knew the place by "Preston Falls".
Unfortunately the task was not so easy relative to the Forestry map. There were no local cartographic files in the Stanislaus National Forest Sonora, California Office. All records were held in the Region 5 Office in San Francisco. I contacted them and found that all records from this era were either discarded in a paper reduction project, or lost in a fire long ago. It is interesting that the 1927 map was one of a series of CDF maps of the same area issued from 1916 through 1941. The name is not shown on the 1916 map, shows on the 1927, 1934 and 1936 maps, and disappears from the 1941 map. Can anything be made of the name's first appearance on the map of 1927?.