"Old Santa Ana" of course, was not where the current City of Santa Ana is now. It was approximately where the community of Olive is today.
Carrie McFadden Ford, daughter of William McFadden, was Regent of the Mojave Chapter in 1929 when the plaque was placed (as shown above). A.W. Kammerer provided the granite boulder. A number of the school's original students were present, and gave short talks.
Phil Brigandi (who supplied the photos and info for this post) writes, "Originally, the plaque was located at south end of old Yorba Bridge. It was moved when the freeway was built in the 1960s. My recollection is that in the early 1970s, it stood between the trees right along the side of the road, and I believe it was moved again when the area was developed."