Is there any standard base / stem? I really don’t think there is. Last time I looked, about a year ago, I saw no standard or universal fitting nubs. Each company has a different size and length of stem. Even different gen Thinkpads require different nubs. I looked specifically because I would like a non-red one for my Thinkpad, but the only option for other colors would be rather pricey 3d printed ones.
Here’s a chart of different sizes needed just for lenovo alone.
Note that these are only lenovo pointer sticks. HP, Dell, and Toshiba are all different from what I’ve seen.