Please keep me informed (really) Chris(at)hawaiidefensefoundation.org
OK so you are going to LOVE this one..

There is zero information on the CA DOJ website, on what must be given to the receiving FFL in CA as "documentation showing you are the current handgun owner", assuming a same-ownership interstate FFL transfer via mail from an out of state FFL to a CA FFL. Your receiving FFL in california will ask you for this "document".
CA DOJ (916-227-7527) informed me on the topic of picking up handgun from your CA FFL:
"Write a letter stating the situation, e.g. FFL in Hawaii is shipping the gun to your designated local CA FFL, for your pickup and continued ownership. There must incude a statement that you are the current owner, and the letter must include gun model / make / serial. Letter must be signed, dated, and then given to the receiving FFL in CA at time of pickup. No notary is required on the letter."
This informal "self attestation" letter basically serves as the CA FFL's "CYA" when they are completing the DROS paperwork and giving you your gun. There are no holds or delays with releasing the gun. You do not have to file a separate Handgun Ownership form with the CADOJ ($19.95) as the FFL's DROS paperwork will take care of that for you.
Must show to the CA FFL at time of pickup:
1. Letter
2. Drivers license
3. CA Handgun Safety Certificate (this is easy to get, you must have one in order to purchase a handgun in CA). It is basically a DMV-style set of 20 common sense questions surrounding gun ownership and you do it right there in the gun shop. Pass questions, pay $20, get your safety card
4. Car registration or other item that shows CA residency
5. Gun lock (trigger lock or barrel lock)
Note this only applies to "same owner" FFL transfers, in which the gun does not change owners, and it is not considered as a sale or PPT of any type. You are essentially mailing your handgun to yourself in California using FFLs, there is no waiting period or BGC. This falls under the CA law that any person moving into CA may bring all of their currently owned firearms with them (with some exeptions for crazy stuff and hi-cap mags), and the "approved" list does not apply.
If you have to mail your handgun to CA, you must mail it to an FFL. You cannot mail it to yourself, and your sending FFL cannot send it to you at your new home in CA. While ATF law says some of this is legal, CA law does not and trumps ATF. Besides, no common carrier will allow mailing a handgun to / from non-licensees, per their own policies. Either the sender or the receiver, or both, must be a licenseee (FFL)
BTW, do you want to know what makes a handgun "approved" vs "not approved" in CA? Basically, the gun maker has to give money to CA along with 3 copies of the specific gun model, and CA subjects the guns to a "drop test". Yes that's right, they simply drop 3 of the same the gun 10 ways to sunday and look for it to break or fire accidentally. So any gun maker would have to do this for every model of gun they produce if they want them to be legal for sale in CA, and that would be extremely expensive.
This "approved" list has nothing to do with a particular handgun being safer than another one, or "less lethal" than another or "less likely to cause crime". It is a complete joke to raise more money for CA and inhibit the private citized from owning their preferred handgun of choice.

Hope this helps, I think this is now the only place on the internet where this information exists accurately in one place.