This is a simple hollow un-balanced (loaded) D6. Similar to
https://www.shapeways.com/product/UJ36TU8E3/balanced-hollow-dice-d6-1-5cm-method-2 but with the center of gravity slightly favoring the side with one pip.
Unlike dice loaded with lead shot or some other heavy element, the effect should be rather subtle. I haven't tested these yet to determine just
how subtle.
Size is 1.5cm.