I just ran into a similar issue with my engine cutting off when putting into gear, although it was in both forward and reverse.
I think the brass nut he's talking about is the male threaded termination of the lower shift cable. The black tube sleeve ends in a brass assembly, with the male threads facing in towards the boat, a 9/16 nut to tighten it in, and a white tube sleeve that sticks out back. It's where you insert the shift cable core to run it up into the boat. This brass piece is the part that needs the special tool from mercruiser because you need such a deep socket to tighten it down, or, in my case, I unscrewed the white tube out, used a normal socket, then screwed it back into place.
I tried adding a picture, but I guess I'm too new to add attachments
https://goo.gl/images/zz9bFW
https://goo.gl/images/szpDr9
If the threads sticking out, as he had it, it wouldn't aligned right when reverse is engaged, effectively stopping the cable from being able to be pulled in as far as it needs to go when putting it into reverse, meeting resistance greater than the springs on the switch actuator, which triggers the cut off switch.
At least that's my take on it
