CHRIS V and other Johson V4 owners

I recently aquired an '83 Johnson V4 VRO (- the VRO part) 90hp. I was wondering what this nipple is for. There was rubber cover over it. Also I need a new set of saltwater anodes for it. What do those run?

On a side note this thing makes my Yamaha sound like a rice rocket.

looks like speedometer hook up

26’ Twinvee Cat
Twin Suzuki 175’s

Capt Ed is correct…the speedometer hookup.