Thanks, but to correct one point in my above comment, I said that the balls were standard pool ball size. I’m pretty confident that I remember reading something two decades back about someone getting sent a whole set of balls by Kensington, and that they were pool ball size. Might have been on the e-zine TidBITS.

I’m getting some conflicting information — the page I linked to said that the Kensington Expert ball is 55mm. There’s a third-party replacement ball here on Amazon, which also says that it’s 55mm.

The page says that the standard pool ball mod “doesn’t always fit”:

The idea of the pool ball in the film is not new: trackball fans have used US pool balls, or billiard balls, of exactly the size 55mm (2.16″) to ‘mod’ their trackball device for years. US size pool balls have reportedly not always fit properly though, so you might need to do some modifications to the device for it to actually work. Good tracking results are not guaranteed, but it will look nice!

However, this page says that a standard pool ball is 57mm:

https://www.legacybilliards.com/blogs/resources/what-sizes-do-billiard-balls-come-in

Pool balls are used to play various pool games, such as eight-ball, nine-ball, and straight pool. The balls are numbered and colored

The regulation size and weight of the balls are as follows:

• diameter of 2 1⁄4 in (57 mm), plus or minus 0.005 in (0.127 mm)

It sounds like Kensington maybe sent the guy a special 55mm pool ball, and the text I’m remembering from 20 years back might have been someone making an error, just assuming that they were standard pool ball size (since they are pretty close), and then the article I linked to making the error of assuming that people were actually using standard size pool balls.

Just didn’t want to mislead anyone trying to do the mod who might wind up with a trackpad and a pool ball that doesn’t fit.