Some months ago, I threw together a small website about the history of Southeastern Huntington Beach. This area was once known as the Santa Ana Gap or Talbert Gap (among other things). This river delta between the H.B. mesa and the Costa Mesa bluffs wasn't incorporated as part of the City until the 1950s.
The photo above is from February 1960. The camera is pointed West along Atlanta Ave, with the photographer standing in front of the current post office near Magnolia. Today, you'd see the Albertson's and Vons' parking lots on the right, and miles of housing tracts on the left.
Link: Talbert GapUpdate 12/20/2022: My Talbert Gap website is defunct, but many of the photos can still be found on my Flickr account.