This is a seven-sided die, with the special feature of clearly showing the value on a flat surface, face-up.
The roller is based on a Reuleaux heptagon.
The numbers are sized such that the same amount of material is present in each face.
About this set
I designed these roller dice to fill in the awkward low numbers : three, five and seven faces. I also have a four-sided roller dice in a similar form, because tetrahedral 'caltrop style' dice are horrid and don't have the value-face-up property.
Although solid, and perfectly servicable, I find the plastic version a little light for comfortable throwing. If you can afford it, I direct you to my 'pepperpot' series (D3 D4 D5 D7) which are the same forms in hollow stainless steel (approximately three times the cost of plastic).
I thought long and hard about the order of the sides for this die. While standard dice have opposing sides summing to a constant, that is impossible here - because there is no facing side.
I elected to minimise the variation of 'sum of adjacent face pairs', using the order : 1 6 3 4 5 2 7. This order has the property of odd positions increasing and even positions decreasing from left-to-right, which is also the case in my five-sided die.