It is a Groco SV series seacock. There is a large rubber cylinder in it with a hole through it. The large handle turns the rubber plug to align the hole with the thru-hull to allow water to pass through. When you tighten the T handle it presses on the rubber causing it to expand and lock in place.
Turning the T handle wonít turn the rubber. To open or close the seacock you need to loosen the T handle and turn the rubber plug with the big handle.
There are no parts available for these. They were discontinued at least twenty years ago. You should be thinking about replacing it next time you haul.