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This drawing may help:

https://www.crowleymarine.com/mercury...25-a26-a27

First you unscrew the surface nut that holds the key switch in place. The mounting plate/bezel is held in place by a black plastic bracket in the back with two nuts. Threaded studs screw into the back of the mounting plate. There are some spacers under the nuts so that they don't interfere with the key switch, so be sure you don't lose them. They are important.

All of the parts for this key switch mounting system are readily available.