New Sloting Plus wheels and other products can be found at Professor Motor at this link.
From Jacques at Sloting Plus:
This wheel, as the previous models, doesn't need insert because this is already integrated in the design. The model is drawn to fit a greater number of sizes, both in diameter and in width, in order to satisfy the customer as well on the esthetic and dynamic point of view.
To make the size identification easier, we have innovated by drawing a channel with a green mark on the 15,9 x 8,5 and 15,9 x10 sizes, two channels with a blue mark on the 16,5 x 8,5 and 16,5 x 10 sizes and finally three channels with a red mark on the 16,9 x 8,5 and 16,9 x 10 sizes . No channel on the smallest 15 x 8,5 size.
Wheels are in a very light and resistant Aluminium-Magnesium alloy with a weight between 0,6 g (for the smallest size) to 0,9 g (for the biggest size) !
The "zigzag" design of the holes is conceived to avoid the abnormal usury of the tire and beyond this it authorize the use of low profile tires that are not adjusted to wheels with bigger diameter holes.