You are fucked.
You "should" be able to put a thumbhole stock on a 10/22, but if that were the case then you should be allowed to place a 22 upper on an AR15 type lower and use it.
There should be no reason why A,B,C or O can't make an AR22. But you know that won't fly in Kalifornication.
Federal law states that you are able to atttach one item to a 10/22 and still be legal.
SO: If you lived anywhere else other then Kalifornication I would tell you that it would be legal to install that thumbhole stock on a post-ban 10/22.
On a pre-ban you could have a bayonet lug, flash hider, pistolgrip and a grenade launcher.
Federal law:
United States Code
TITLE 18 - CRIMES AND CRIMINAL PROCEDURE
PART I - CRIMES
CHAPTER 44 - FIREARMS
SECTION 921
Assault Weapon:
(B) a semiautomatic rifle that has an ability to accept a detachable magazine and has at least 2 of -
(i) a folding or telescoping stock;
(ii) a pistol grip that protrudes conspicuously beneath the action of the weapon;
(iii) a bayonet mount;
(iv) a flash suppressor or threaded barrel designed to accommodate a flash suppressor; and
(v) a grenade launcher
Kalifornication law:
ROBERTI-ROOS ASSAULT WEAPONS CONTROL ACT OF 1989
12276.1. (a) Notwithstanding Section 12276, "assault weapon" shall also mean any of the following:
(1) A semiautomatic, centerfire rifle that has the capacity to accept a detachable magazine and any one of the following:
(A) A pistol grip that protrudes conspicuously beneath the action of the weapon.
(B) A thumbhole stock.
(C) A folding or telescoping stock.
(D) A grenade launcher or flare launcher.
(E) A flash suppressor.
(F) A forward pistol grip.
(2) A semiautomatic, centerfire rifle that has a fixed magazine with the capacity to accept more than 10 rounds.
(3) A semiautomatic, centerfire rifle that has an overall length of less than 30 inches.
Senate Bill 23 Assault Weapon Characteristics
(1) "Magazine" shall mean any ammunition feeding device.
(2) "Capacity to accept more than 10 rounds" shall mean capable of accommodating more than 10 rounds, but shall not be construed to include a feeding device that has been permanently altered so that it cannot accommodate more than 10 rounds.
A large capacity magazine is defined as any ammunition feeding device with the capacity to
accept more than ten rounds but shall not include any .22 caliber tube ammunition feeding device, or a magazine that has been permanently altered so that it cannot accommodate more than 10 rounds.