The primary difference among our 23 Gauge Headless Pinners is the maximum fastener length the tool can drive. 1-3/16" for the P630, 1-3/8" for the P635, 1-3/4" for the P645 and 2" for the P650. All these tools can drive pins as short as 3/8". However, in the P645L and P650L, which are equipped with the Auto Lock-out feature, the trigger will lock by default if 3/8" pins are used. The tool can drive 3/8" pins as long as the Lock-Out Override is depressed.