I was able to find my set from storage and have been using them for a few days now. It really helped to loosen and relax my finger joints, which are usually stiff with a slight discomfort feel to them. Since twirling the balls for 4x 10 min sessions a day, I do not feel the discomfort anymore.... a dramatic improvement!
Mine are chrome plated iron (magnet sticks to it). They are 1.95" (49 mm) diameter measured with a digital vernier. They are slightly different weights, not sure by design or manufacturing tolerance (probably the latter). one is 214 gram 0.47 lb, the other is 226 gram 0.5 lb. They are hollow, with a chime that rings with the balls collide.
Google reveals these come in different sizes. Mine is in the large size range. I do not have big hands and these work well for me.
Thanks to this thread I am using them again. Last time I touched them was probably more than 20 years ago. I will try to make it a daily regimen.
I wash them with hot water + soap before use to keep the germs down. warm steel feels much better in the hand than cold and is better for blood circulation.