With the "-8" series K frame of the Model 66 and the new 69 featuring an improved crane to frame lockup, more thumb pressure pressing simultaneously inward and forward is necessary to actuate the thumb latch to open the cylinder. Traditional oem latches do not have enough contact surface and the side of the thumb tends to slide off before opening the cylinder. The upgraded TK design remedies this issue while enhancing the appearance of the revolver. A welcome improvement as well on other modern S&W s.