It *probably* unscrews, but I'll let an actual REKATian answer for sure.
To anyone in California: DO NOT ditch the thumbstud, for legal reasons. And I would strongly recommend against it to anyone in the US. The gent asking is in Asia and I have no idea the rules he's operating under but here, it's all too easy for a cop to decide the Sifu is a "gravity knife" if the thumbstud is gone.
Santi, my only advise is make sure you're not running into a similar problem.
Jim