PW-127M is just a tweaked -127F with better 1-Inop power, about 5% extra for take-off and max continuous. The 72-500 can and are being fitted with the M. The 72-600 has more improvements in cockpit and cabin technology, the airframe and performance are much the same as the -500 with a little better short field and payload capability brought on by the improved single engine performance.