On the underside of the stop down mechanism, you have to take it apart to get at four screws that hold the bayonet mount in. Unfortunately these screw holes do NOT line up any where usable on the M42-->AF adapter, in fact, they line up with the mount itself! Drat!! Be careful taking the stop down mechanism apart. It isn't hard to take apart or put back together, but there are a number of small parts that you don't want to lose or forget where they go. Take a photo if you need to.