Pinned & Recessed
A pinned barrel is one in which a pin runs through the frame to secure the barrel once it has been screwed into place. The pin positively prevents the barrel from turning out of place from the bullet's torque.
A recessed cylinder is one in which the chambers are counterbored, allowing the cartridge to fit flush in the cylinder. This provides protection in the event of a casehead rupture, but it also looks more finished.
Neither of these refinements are truly necessary in a revolver.
Smith & Wesson discontinued them both years ago in a cost cutting measure to insure the company's survival. These refinements endure, however, as indicators of a revolver that was skillfully crafted by artisans who loved their work.
Pinning of barrels and recessing of cylinders ceased in 1982.