It was made by Groco. They discontinued them more than twenty years ago.
There is a Tee handle on one side and a lever handle on the other. There is a large rubber cylinder inside with a hole through it.
To operate it you loosen the Tee handle and rotate the rubber cylinder with the lever handle. When the lever handle is vertical, water
can come through. When the lever is horizontal water
can't come through it.
Position the lever where you want it and snug down the Tee handle. The Tee handle compresses the rubber cylinder which causes it's sides to expand and lock the cylinder in place.
If it's old enough and used enough, the rubber goes bad and they will no longer seal. When left in the closed position for long periods of time, the rubber can bulge into the openings locking the cylinder in place.
These aren't bad seacocks as long as the rubber is good. When the rubber goes bad, replace it with a newer seacock. Parts
are no longer available.
See the plate on the lever handle side? There are two bolts that hold that in place. Remove the bolts and the rubber cylinder will come out.