It's in the California Penal Code, Section 12078(t)(2)...
The "Subdivision (d) ... of Section 12072" referenced in the above quote is the section of the California Penal Code requiring people in Califonia to complete most firearms transfers through a California Licensed Dealer. So 12078(t)(2) is saying those transfer requirements shall not apply if the firearm is a 50+ year old C&R long gun.
Subdivision (d) and paragraph (1) of subdivision (f) of Section 12072 shall not apply to the infrequent sale, loan, or transfer of a firearm that is not a handgun, which is a curio or relic manufactured at least 50 years prior to the current date, but not including replicas thereof, as defined in Section 478.11 of Title 27 of the Code of Federal Regulations, or its successor.

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