Mechanical mouse turned upside down. Has the usual wheels and optical sensors inside. Track ball is a little small. The side buttons take some getting used to and don't click firmly as they used to, but all said and done, its a decent functional device at the age of 25. Perhaps that's because trackballs don't collect lint like a mouse that puts the ball on the pad.
You need to get the correct PS/2 to 9 pin serial adapter. Didn't work with the first one that I tried. They are not all wired the same.