The OSS experimented with a lot of…unorthodox weapons during World War Two, and one of their overarching goals was a weapon with a 100 yard lethal range but without flash or noise. To this end they experimented with a number of suppressed firearms as well as weird stuff like various crossbow designs, silenced dart gun pistol conversions, and in this case a CO2 powered dart gun. It was code aimed the Flying Dragon, and first mentioned in documents in 1943. In the summer of 1945, 15 were manufactured, and 12 of these remained in OSS stocks at the end of the war.
In July 1945 testing, the Flying Dragon was found the be the second-quietest option (the William Tell crossbow was quieter, at 66 decibels to the Dragon’s 69 decibels). However, the testing board noted that a simple suppressed .22 pistol was pretty much just as good, and quite a lot cheaper (and more reliable, I would expect). The problem with a dart gun like this one is that if it is not reliably lethal, the whole point of its silenced is lost. Anyone shot by that big dart and not killed by it (which would require a pretty significant muzzle velocity) will immediately start making a heck of a lot of noise. OSS investigated options for poison on the darts to give the weapon the necessary lethality, but was unable to find a suitable solution. This led to discussion of using a small hypodermic syringe as a projectile, an even less practical idea – but this was the freewheeling OSS, where such things were not uncommon to consider.