These are the cartridges in question (illustrations courtesy of Chris):
A tentative identification can be found in U.S. patents no. 3,141,412 and 3,311,061 assigned to Elroy C. Roehrdanz of San Francisco, California. The first patent was filled May 18, 1961 and describes a sabot formed of plastic (Polypenco Nylon 101 or an equivalent) designed to use commercially available bullets. It mentions sabots for .243 and .257 diameter bullets. The patent drawing shows a similar looking cartridge.
The second patent was filled June 25, 1964 and describes a different sabot construction made of Delrin or Lexan with an outside diameter of .3115, inside diameter of wall of .244 and a bullet diameter of .243. It further says: “Such a structure is very effective with a projectile made of steel or some other hard material. Obviously, if this structure is used with a soft lead core bullet which tends to bulge when subjected to the explosive force, the base of the bullet will tend to hold the sabot from separation” (sic).
As you can see, there are several similarities with some of the cartridges in question, although I admit this is far from being a conclusive identification. By the way, does anyone have colour pictures of these cartridges?