^ can't help you with the CZ threading question, but i'm sure there are many here that can (pun intended).
being left handed, most thumbholes to me are uncomfortable, but having an ambi thumbhole on my 455 aics airsoft conversion, i'm still on the fence with thumbholes. as my other rifles don't have thumbholes, i think i may go back to a traditional or target stock grip so that there is some muscle memory easily transferred without having to slightly modify my grip to accommodate.
to me (JMO) the thumbhole does provide a more straight back into the shoulder pocket presentation, but also introduces some gripping issues that are prevalent in pistol shooting. also carrying it feels awkward to me, and i guess that even though there is a hole and cavity there, the stock seems bulkier and heavier as the wood around it has to compensate for going around that hole and cavity. if that makes any sense, it could just be me.