Not to be confused with the California Department of Water Resources' manmade Castaic Reservoir in Castaic, this is Castac Lake in Lebec. (It was often spelled "Castaic Lake" before the fake one came along in the 1970s.)
Undated real photo postcard, circa 1930s. Printed description: "No. 7 — a beauty spot, Lebec Cal." We don't know about other "numbers" in what is apparently a series.
Castac Lake was indeed a beauty spot, and one day soon, a lot more people might be able to enjoy it up-close and personal. The Tejon Ranch Co. plans to surround it with 3,450 homes as seen here.