I did a little poking around and found the following excerpt at the California Department of Consumer Affairs Bureau of Automotive Repair website
3.) Verify that the vehicle conforms to U.S. and/or California emission control standards.
Check for conformity
The vehicle must meet either U.S. Environmental Protection Agency (EPA) or California standards. It is legal in California to buy or sell a used vehicle that meets only federal standards.
Look for an underhood label
1971 and newer vehicles have label showing that they were manufactured to meet either California or U.S. standards. The label is affixed in the engine compartment or in a readily accessible location. At the top of the label are the words VEHICLE EMISSION CONTROL INFORMATION and the corporate name or trademark of the manufacturer.
Gray market (direct import) vehicles not originally designed to meet U.S. specifications may require major modifications and must be certified by the Air Resources Board before registration in California.
If you encounter any vehicle without a label indicating that it meets either U.S. or California standards, watch out. Beware of unsubstantiated claims that the vehicle meets the standards or that it can be easily equipped with pollution control devices.
So, it does encourage the buyer to use caution if this label is not present, however it doesn't seem to explicitly state that the label is required to be present. Of course, this has more to do with being able to sell the vehicle rather than pass emissions standards, which I hope California is not relying on some old under hood label to determine.