This is the structure that local sources know as the "Preston Homestead". It is a very large (30' x 60') concrete slab with a stone hearth built about one third of the long dimension in from the Eastern end of the slab. The NE corner of the slab shows in the foreground and the SW corner of the slab is in the background near the tree line. The collapsed hearth stands on the left with the top fallen over to the far left. The opening in the hearth faces to the right in this picture. There are no visible indications of walls of any kind. The hearth does have remnants of vent piping which indicate the top of the vent pipe would be 20 ft above the ground. In the refuse piles around the site there are shelf units that might have been wall mounted.
There are a number of cooking plates and pans remaining on the site that might be period dated. Even more useful are a few refractory bricks within the hearth structure that have the name "LINCOLN" stamped on them. Someone told me that there was a brick factory in Lincoln, CA , near Sacramento. I used the web to locate the Public Library in Lincoln and called the reference librarian. She told me that there was a brick factory in Lincoln, by the name of The Gladding-McBean Brick Company. Within a week or two, I was put in contact with Angelo Simone, a prior employee of the company who indicated that these bricks were indeed made by Gladding-McBean, and were discontinued in 1928.
Angelo says that the company used kilns to fire clay sewer pipe. Prior to 1921, the process was called a "beehive kiln" where the bricks in the walls of the kiln were consumed and then sold as "clinkers". In 1921 the "tunnel kiln" process was invented - it was a continuous process - and they needed refractory bricks for the walls of the kiln that would last. They acquired some clay mines near Ione, CA in the early 1920's to get a source of refractory clay for such bricks. They were so successful that they began to sell the refractory "LINCOLN" bricks themselves.
So the conclusion is that these bricks in the hearth were made by Gladding-McBean sometime in the period 1921 to 1928. They could have been obtained and used at the site later than 1928 when the manufacture of the bricks was discontinued. However, the hearth could not have been built before 1921 because the bricks did not exist.
This earliest possible date is some 31 years after the death of A. B. Preston and at least 4 years after the death of his son Howard. It appears that the site cannot be the "homestead" of either Preston. Thus the site is either unrelated to the name, or it is not named for A. B. Preston of Sonora. I lean toward the former position.
The age of the bricks is the first reason I believe that site is not related to the Preston Falls feature on the maps. The second is the location of the slab and its accessibility. The site is at least three miles away from the nearest road, meaning the materials must have been brought in by pack trains. Where would they have come from? There is no access whatsoever from the East, where the majority of the falls along the river are located. The falls commonly referred to as Preston Falls is only a few hundred yards east of the slab, and is only the first of a series moving upstream to the East. Obviously then, the access is along the trail that leads from the bridge crossing the Tuolumne River at what is known as Early Intake.
I don't think it is a coincidence that the bricks can be dated to 1921 or later, and that construction of the Hetch Hetchy Project, and in particular the Early Intake Powerhouse, peaked in this same time frame. Concrete building materials were plentiful in the area. I believe that this site is somehow related more to Hetch Hetchy than to Preston Falls, and may have been used as some sort of recreational facility during that era.