They are just vintage. There are no advantage to a duckbill. They work OK. But there are reasons why you don't see them on any modern gear (rebreathers, single hose regulator exhaust, SCBA valves, other breathing devices, or many other low pressure differential check valves, they normally use mushroom valves). BTW, modern mushroom valves are very different and have evolve a lot from the early flat rubber valves. There is a lot engineering design and analysis into the geometry of what looks like a simple mushroom valve.
The vintage duckbill actually have a bit of a constriction to the flow. The rectangular cross section of the duckbill has a reduced flow area, but in a double hose regulator is not normally an issue.
These ones, made out of silicone are much better than the originals and will not bond to the inside of the metal horns. The silicone is flexible and will not deteriorate like the originals.
Buceador con escafandra autónoma clásica.