You might wait to hear from one of our Cali member's, but I don't see why it'd be an issue.
It's a part of a firearm that is legal in CA. A quick call to your sheriff's dept. or other local law enforcement agency could provide a definitive answer...
just looked up some info. what you are ordering is an accessory. gun part restrictions are like on numbered frames, etc.
as mentioned, you can ask the local pd or ffl.