Ok I also bought that, mine didn't have one being a 1988. HOWEVER....I don't know that I will install it because, I'm not sure that I will be able to get that plug out and also, the next time I have to replace the shift cable, I doubt that the shift cable retaining nut will come out of the pivot housing, so I will probably wind up replacing my original with a spare fresh water used one I bought.
Also, while I think this is a great idea, what I have done for over 20 years in salt water is simply this:
each season drive gets pulled
I clean out any crud that is behind the bellcrank
Grease it up good with marine grease
and every 5 seasons or so, replace that little O-ring that goes on the bellcrank shaft, that's what keeps water from the exhaust area out of the bellcrank area. If when you pull the drive, the grease in there looks milky, that oring is probably leaking/all flattened out.
I also clean off the sealing surfaces of the drive, and pivot housing, and coat both with OMC gasket sealer, same with the gasket. This keeps water out pretty well, and keeping water out, and the bellcrank greased, will keep your Cobra shifting like it should, with just 2 fingers.