Aww yes it is. It is not "your name to gun S/N" indexed, but it is "gun S/N to name" index (at least last FFL transfer name). You are correct that 4473s are held by the dealers (while in business), but that doesn't make it not a registry just distributed. This is used all the time to find the owner (via FFL) of a gun.A federal form 4473 is not registration since the form lives with the dealer.
When people talk about a gun registry they really mean changing, replacing, or adding a new registry that allows them to index by name (i.e. John Dow owns ….) and yes this is already the case for title 2 items.
But this is way off topic. At the core of the question, if you bring anything into CA as a new resident (including moving back) and it’s not already in the CA registry you will need to add it. This technically includes any grandfathered gun that you took out and are now bringing back in, but it would be up to them to prove that you took it out and brought it back vs just leaving it in CA while you were gone.
We are talking about the DROS registry which started in 2000. I'm not sure but the 1931 date sounds like the federal NFA title 1 section, but if your right it just makes the grandfather date evan older.Handguns purchased/transferred through a CA FFL dealer have been registered since 1931.