Jeff and I posted some stuff here and you can see a pic here
Depending on area of Los Angeles, you will see the valves installed on the customer side of the meter, braced to the wall etc.
Can a house be sold w/out one? Would be unusual if the SFR is within the jurisdiction of LA City (SFV, portions of Los Angeles etc) Last I checked this was enforced (to a degree) at point of sale, via retrofitters etc. Also, some information here
If you’re not sure about the area you’re working in, do some research. It can be bit tricky, but in a nutshell if a city within LA County doesn’t have their own building and safety, the city may be under the guise of LA City (thus requiring a seismic valve) or in a county or other jurisdiction that may not.
A few phone calls here and there should solve.