These hinges are used on drop front desks and secretaries. They are mounted barrel up and mortised into the lid and case so that the leaves of the hinge are flush with the surface of the wood. They are made from solid brass, pictured in antique, and available in four finishes.
Every now and then, we get requests for desk hinges that can be installed with the barrel down. The mechanics of the way a hinge works prevents this from being possible. There are hinges designed to be mounted this way, but they are not used for desks, because they will not fold up as far as a desk hinge needs to fold. Mounting a hinge with the barrel down on a desk creates many other problems, as you will need to carve away a tremendous amount of wood, right in the rabbet where the wood is less thick.
The writing surface of a desk is actually nowhere near the hinge and the hinge never gets in the way, even though the barrel is up.