Normally in the upper right corner of the transom. Unfortunately, it is often not visible in older boats. It is also somewhere hidden below, maybe behind a bulkhead. Hidden to help ID a boat in case of theft. Of course, this applies to boats built after 1972.
I’ve reached out to someone but have not heard back. To date, I have not found any information indicating the Cal 3-30 (version 3 of the 30, designed for 3/4 ton IOR racing), was built after 1974.