You’ll see plenty of evidence on the street of the East Bay Municipal Utility District. Their 8-inch mains are marked with these ubiquitous caps.
Much rarer are the caps for EBMUD’s predecessor, People’s Water Company. It served much of Oakland and points north for the first time, an important ally for developers, but went bankrupt more than a century ago.
Here’s another artifact I found up in the hills.
In 2013 it was the subject of a post on my other blog, Oakland Geology.