Price is $.25 each, shipped free with any other order. (U.S. sales only). I can't find any reference to Flechettes this exact size, so maybe this is something that is missing from your collection.

Prarphrased freom Wikipedia:

During the Vietnam War the United States employed 12-gauge combat shotguns using flechette loads. These plastic-cased shells were issued on a limited trial basis during the Vietnam War. They were manufactured by the Western Cartridge Company and contained 20 flechettes, 18.5 millimetres (0.73 in) long and weighing 7.3 grains (0.47 grams) each... The flechettes had flatter trajectories over longer ranges than spherical buckshot, but combat effectiveness did not justify continued production.

Smaller flechettes were used in special artillery shells called "beehive" rounds (so named for the very distinctive whistling buzz made by thousands of flechettes flying downrange at supersonic speeds) and intended for use against troops in the open – a ballistic shell packed with flechettes was fired and set off by a mechanical time fuse, scattering flechettes in an expanding cone.

During the Vietnam War 105 mm howitzer batteries and tanks (90 mm guns) used flechette rounds to defend themselves against massed infantry attacks. The ubiquitous 106 mm M40 recoilless rifle was primarily used as an anti-tank weapon. However, it could also be used in an antipersonnel role with the use of flechette rounds. The widely used Carl Gustaf 8.4cm recoilless rifle also uses an Area Defence Munition designed as a close-range anti-personnel round. It fires 1,100 flechettes over a wide area.The US Air Force used 2.75-inch (7 cm) rockets with WDU-4/A flechette warheads.