In 2005, the CA AG provided the informal guidance in the letter located here:
From this, all magazine parts (including bodies) can be replaced. Note the cautions in some of the responses. Local DAs have power to interpret and prosecute. It would be prudent to keep records of changes.
You don't have to prove your innocence, and I agree with not talking freely to the police, as you might inadvertently admit to a crime. But this letter rides in my range bag along with other documentation. While it's not something I would immediately offer in a discussion with LEOs, it is somewhat comforting to know I have something with me which may help mitigate the issue, if needed.
NOTE: THIS IS NOT A "GET OUT OF JAIL FREE CARD
". California has changed Attorney Generals since the issuance of this letter (the AG in place in 2005 (Lockyer) is now the State Treasurer, and the AG after him (Brown) is now the Governor), and the new AG (Harris) does not appear to have specifically endorsed (or refuted) the 2005 letter.