Over the last decade, adaptive tendon driven devices have gained an increased interest from the research community for their lightweight, compact, and affordable design features attributed to the utilisation of underactuation, differential mechanisms, and structural compliance. Although adaptive tendon driven devices are capable of efficiently executing stable grasps under significant object pose uncertainties with simplistic control algorithms, they lack the controllability over individual fingers in comparison to traditional fully actuated designs. In this paper, we focus on the development of a selectively lockable differential mechanism that is powered through a small and low torque servo to provide increased autonomy to highly underactuated and adaptive prosthetic hands, without compromising the weight, cost, and compactness of the device. The proposed prosthetic hand is experimentally validated through four tests: i) grasping posture and gesture execution experiments, ii) grasping experiments with everyday life objects, iii) force exertion experiments, and iv) Electromyography (EMG) based control of the prosthetic hand.