This is a three-sided die with 'fudge' markings (+, -, 0), with the special feature of clearly showing the value on a flat surface, face-up.
The roller is based on a Reuleaux triangle, with the corners slightly clipped to improve its rolling characteristics.
The symbols are sized such that the same amount of material is present in each face.
I believe that this roller form is superior to a cubic die with paired values, because an unscrupulous player able to throw a die such that it rotates on one axis can reduce their exposure to undesired values. (This is the case for all possible layouts of a d6, which stems from the fact that four is not divisible by three).
About this set
This metal form is a 1mm stainless steel shell with a hollow interior. Together with the rounded shape, this gives the dice a very pleasant feel and weight for rolling. They roll extremely well, and stop well centred on the faces. The ends have the round holes which gave this set of dice their name.
The symbols are quite clear enough to view at a glance in plain stainless steel. Polished bronze steel would be perhaps slightly less visible. If the numbers are not clear enough for you it is straightforward to enhance them by adding a small amount of a contrasting paint, then wiping off the excess.
This die form is also available in colourful plastic, and a set of four in plastic. I also have listings for similar odd-numbered dice with digits (three, five, seven sides).