Vintage50 - I have a set of CS3 spikes in hand and believe them to be the same size as the CS3.5. There are two sets, one pointed (for carpet) and the other rounded (for hard floor.) Both are 1/4" diameter. Pointed are 2" long and rounded are 1.25" long. They slide into holes with plastic entry collars in the plinths, two at the rear and one at front center. My recollection is that we added some texture to the 3.5. Instead of being plain, they have a tiny O-ring in the center and some parallel grooves off center to keep them in the sockets, and to provide more purchase to remove them.
Sorry I don't have 3.5s on hand until they someday emerge from storage.
