Re: 1956 30hp johnson javalin
It should be the "cutout lead assembly" which grounds one or both ignition points to shutoff the motor. Look at diagram OM9-27 on page 151 of the service info I PM'd you for more info. However, that is a generic diagram covering several model years of this series motor and may NOT be exactly correct for your motor. In that diagram, two black wires exit the magneto, one to the CS (cutout switch) and one to the MS (mercury switch). The CS is a safety device that protects the motor from overreving while not under load by sensing crankcase vacuum/pulse signals. The MS is installed on the throttle shaft and prevents the motor from being started at a high setting.
FYI, I looked through the parts catalog for your motor and cannot find either the CS or MS listed and do not understand why. I am not an expert and maybe another forum member will post an explanation for all of us. Meanwhile, I am going to look for a link to another wiring diagram that may help. Good luck!
EDIT: The link below will get you to a wiring diagram identified as 1958-1961 but it should be mostly correct for your motor. Just use it for reference/info to help you understand what you are dealing with.
http://i155.photobucket.com/albums/s311/frankr_photos/Wiring1958_6135_40hpwithoutgenerato.jpg
