For all those out there that think this is rocket science, it isn't. Here's one of the many references one could use:
https://www.pewpewtactical.com/featureless-ar-15-rifle-grips-stocks-muzzle-devices/
Starting off, the M1 carbine is not a potential problem under federal laws, like barrel length or selective fire/full auto, etc. If it is, then it's not as the OP described it. It wasn't previously registered in Ca. as an assault weapon. It's not one of the Ca. specific listed by name weapons. Neither it's barrel length nor overall length are an issue. Nor is it's caliber, it's not a ".50 BMG." Magazine detachable? Yes. Mags must be 10 rounds or fewer. But that wasn't the question. Bullet button? Not if it's a standard M1 carbine.
So, what are the features that define a featureless rifle? Does it have any of these?
Pistol Grip - Nope.
Thumbhole Stock - Nope.
Folding/Telescoping Stock - Nope.
Grenade Launcher or Flare Launcher - Nope.
Flash Suppressor - Nope.
Forward Pistol Grip - Nope.
It's a classic "featureless" rifle. Now just for some clarity, for all you guys that keep wanting to bring up bayonet lugs. A bayonet lug is not a factor in California.
Where could you get in trouble? Friend at work was late in getting a build done before the beginning of the year. He has to go featureless now. So what are the tricky, sort of, spots that might be a problem?
Until just quite recently, the proposed regulations/definitions that go with the laws weren't available for review let alone finalized. So what might be a "good thing" in the regs? They have actually filled in some of the unclear (but not necessarily fixed all the lack of clarity) features carried over from existing laws. This is good because when you draw a line in the law/regs, it is easy (or should be) to know where a feature, item, device, etc., is in when it comes to compliance. Like barrel length - they describe how barrel length is to be measured. Easy to enforce, easy to comply.
Examples of an improvement. They defined, like it or not, and perhaps still not well enough, what would constitute "disassembly" when it comes to dealing with "fixed" magazines. They now specifically define a stock. Before the question was out there as to whether a stock, like a Thordsen, might be described as a pistol grip. The issue there is because a pistol grip (for California) is partially described by where the web of the hand (between thumb and forefinger) goes. Now, "stock" is separately discussed and that's not an issue. (Note that a Thordsen and similar stocks replace the pistol grip and original stock, so to speak. There are still requirements in place covering certain types of grips and stocks to be observed.)
Still lacks clarity? Flash suppressor. They struggled with the definition last time around and it remains problematic.
So where could the friend get in trouble? Deleting the usual flash hider/suppressor and adding a brake that somehow triggers one of the Ca. factors. Retaining a non-folding/non-telescoping stock and not addressing the "pistol grip."
So what might be an area that gets bandied about incorrectly? Bayonet lugs - no need to restrict oneself from having a bayonet lug.