It is NOT a private party transfer. Those are clearly defined as being between two residents of California, and those have to be performed by both of them meeting at a 01-FFL to perform the 4473, DROS (including DES entry), and 10-day wait.
Logically, a 01-FFL can buy any handgun from out of state, independent of the roster. Many gun dealers in California have a special showcase for "LEO only weapons", where they sell non-roster handguns. Those obviously come from out of state. Whether the non-roster-gun comes from a private individual, or from a distributor like RSR or Davidson's doesn't matter to the law: the 01-FFL can buy a non-roster gun from out of state, take it into his inventory, log it into his book. However, he can not sell it to normal state residents, only to exempt people.
I have NO idea about how the separation between personal collection of an individual who has a 01-FFL license, and his FFL-controlled inventory works.