An article in the 2 Sep 1971 issue of the Fallbrook/Bonsall Enterprise "Wartime Landmark -- Rainbow residents in the 1940's used this lookout, now the Knott property, to spot aircraft during World War II." I believe this is 2050 Rice Canyon Road. I can see the shadow cast by the tower on the 1964 aerials from https://historicaerials.com/viewer. Another article in the same paper dated 3 Feb 1994 says the "Tower House" was built by the Marine Corps in 1955 and was used to watch for Russian aircraft until 1960. The article documented the removal of the tower in Feb 1994.