"Constructive Possession" is the new term they are tossing around with regards to situations like this. (They used to call it "Constructive intent" but with intent being hard to prove, the decided to rename it "Constructive Posession.")
So, you own a complete AR. Now if you have an "orphan" short barreled upper, and no legitimate lower, either a pistol lower or a registered SBR, for it to mate to, then you have "Constructive Possession" of an Illegal Short Barrel Rifle. It makes no difference if you have ever mated that Short barrel upper to it or not.
The chances of getting pinched on this is probably pretty slim, however, it is a felony that carries with it a possiblity of 10 years in club fed.
Frank CPL USMC: There are many states besides California that require registration of pistols.