A good core retunr for a factory overhauled Slick Magneto is going to be a current series 4300 or 6300 Slick Magneto. If a new Slick Magneto is purchased, then older series Slick Magnetos can be considered for credit on a case by case basis. 

Check the "Detail" tab of the Slick Magneto that you are interested in buying and the Slick Magneto listed with a "Type: Impulsed" will come up with an impulse coupling.

There are no bill backs on a core return that is a current series 4300 or 6300 Slick Magneto. 

According to the Slick Magneto by Champion Overhaul Manual L-1363F, the magneto should be overhauled at TBO, lighting strike, prop strike, sudden stoppage of the engine and immersion of the aircraft.