The ballstretchers are split-rings which you open and close with a hexagon key (which is supplied with every ballstretcher). They come in different heights and the most popular ones come in two different internal sizes.
The weight of this stretcher is 680 grams. Internal diameter is 34 or 38 mm
Tip: think about buying two or three small ones instead of one big one: you can stack them on top of each other to get the big one, but you can also use them separate for a lighter result!