I _think_ that the trackballs are 40" Omnikin balls.
http://www.athleticimpact.com/ba552.html
http://www.robbinssports.com/sportin...7_250_255.html
http://www.sportsdirectsource.com/products/359
...but as they do not have the _weight_ listed, it's hard to say for sure. The 48" Omnikin ball our PE department has is approximately the same weight as the trackball, but might actually be a little lighter. The covering sack is virtually identical except for color, as is the appearance of the bladder material.
Patrick