Now you have me thinking as well. I remember seeing the dots on many tires, just not recently (last couple of years). Could have swore that they were on the PR2,3 &4's that I used to run. Certainly not on the PIrelli Angels, Metzler 01, or BST31's that I have last mounted.
I have gone through many sets of PRs, PRIs, PRIIs, PRIIIs and PR4GTs, none have had the dot!