I am certainly not suggesting anyone rely on me for legal advice, to be clear.
But no, my understanding in CA is unlike the Feds, California does not have the concept of constructive possession as it applies to assault weapons. As such, separated parts cannot constitute a CA AW, unless the lower is already registered as such, or said lower is on the list of named CA AWs.
It is technically not an assault weapon in that configuration; however, depending on the DA, they may still come after you under PC 12280(a), stating that you are attempting to possess an assault weapon. The sticking point for them is showing intent, but they have convicted on possession and research of how to assemble an assault weapon.
They certainly could charge someone, and maybe even win; I’m not suggesting anyone rely on this as a defense, but that is what the law states. Short of any other aggravated reasons to charge you, it’s unlikely a DA would have any interest.