These are band new, drop-in, D-system, twin-lug American Sankey beer keg valves. They come equipped with o-ring.

Keep a few of these backup valves on hand to minimize out-of-service kegs.

These are 1/2 barrel (15.5 gal) standard keg length (21.75″) but can be cut down to fit smaller capacity kegs.
Sold in packages of 10 valves. For example, ordering a quantity of 2 will give you 20 valves.