No, there is no provision for registering or exemptions. Once that section of the penal code went into effect 1/1/2000, that's it.
Tax stamp and 922R are federal regs., not state.
Even though that stock as you purchased it does not have the capacity to telescope, I'm surprised that it was sold in Ca.. It's the first Iv'e ever heard of.
Maybe it's a recent addition of Ca. available M4's since it's unable to scope in and out. Has the "look" but doesn't work.
Yeah, the only other legal options for a Ca. resident are the straight or "field stock" or, the more commonly found non collapsable pistol grip stock.
Better you found out now before you spent $$$$.