Not familiar with these. What do they do?
It is a remote drain plug. Good for those boats that you really have to get down on your knees to access the drain plug. It has a cable that is attached and a rotating switch that you mount somewhere in your splashwell so you can plug it or unplug it while standing at the transom. Works a lot like the livewells in most boats today.
Extremely useful if you launch your boat then realize you forgot to put the plug in. Just walk to the back deck and turn the switch.