Good observation, Jeff! Now that I look more closely, it appears that Jack's right arm is similarly articulated (less clear on the left arm, but apparently jointed there too).
One could conceivably email Mr. Ehrenstein and ask him about his source (I once emailed him on a completely non-BBM-related matter, and he replied to me), but I'd be kind of nervous to do so, considering his intense antipathy to this film.