The item in question is #26 in the late 115-6 parts break-down.
www.marineengine.com/parts/mercury-outboard-parts/115-6-cyl/6436123-and-up-usa/end-cap-manifold-and-exhaust-covers
BUT,...
IMHO
It is like the Doppler Radar Warning Device (radar detector) in a car - it only tells you when you have just been had by the police radar. In other words,...When the warning horn sounds - your Merc is already internally damaged significantly.
Just to clarify,...The 1500 inlime-6 has a CDI ignition (capacitor driven ignition)
If your 90 HP inline-6 has (6) coils on the starboard side,...That ignition is referred to as an ADI ignition or - (alternator driven ignition).
If your 90 HP just has (1) coil,...Consequently it is the same (CDI) ignition as the 1500 inline-6.
The ADI ignition was introduced in the 1979.5 models of the inline-6.
This same ignition (ADI) was first used in the 1975-1977 4 cylinder Merc 850.
It continued to be used in later model 80 4 cyl, 75 4 cyl, and the inline-6 through 1988.
It was also used in the early Looper L-4s ('88-'92) & the early v-6 Mercs.
The cool part about the ADI ignition is that there are a lot of ignition failures that can fail, and you can still limp back to the dock.
If you have the CDI style ignition (CDI),...When it fails it is time to tell your Wifey-Pooh to man the paddle and make her way to the boat launch while you relax and drink all the beer!!!
doc F