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On a new construction build down the street, I am interested to know what this is and what it does? We are in California on public water if it makes a difference.
A placeholder for the water meter [or a secondary water meter to differentiate internal (& sewer) water from external (just water) uses] is one possibility.
Or for some other (not presently installed) device that mounts with unions, such as a Pressure Reducing Valve (PRV.)
Looks like it would be to electrically isolate the home's piping from the city's.
Required, but never shown