Mystery cap

Any ideas what this cap is for? House was built in 1920’s in San Diego. No basement.


https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcR3fV19nC2OyeGHLZLpvqV45slra-MZ5MFT1oixq7o4p48u3qFnrA

Awesome! Thank you.