Cistern in yard -- part of Camp La Mesa?

Hello-

Not sure if this is the spot to ask, but thought I'd give it a shot.  : )

My mom lives in Fletcher Hills (Annette Way, El Cajon, CA 92020) in a home which was built in the 1950’s.  I was in her yard digging, getting ready to put in a new fence.  I was removing some old bricks which were about 8” wide.  I dug down about 12” and hit something solid.  I found solid cement…….then what appeared to be an opening.  I stuck a camera as far as I could reach and took a couple pics.  Thought it was an old well or something, but after some research I think it appears to be an old “Cistern.”  Although I really have no idea.  Was trying to figure out what was on the land prior to my mom’s home being built.  Couldn’t really find anything other than Camp La Mesa in the area (An Army camp from back in WWII). But it’s hard to tell exactly where this camp was located.  Some old maps look like there was a train track near, but can’t find any info.

Would you happen to know where I might find out info of what was on the land way back.  Is there records I could research??  Can you let me know either way please?