These designs are a set of rings to measure the size of your finger and see what ring size you are most comfortable with. The files are designed and named after the US chart but you can refer to the ring size chart in the illustrations to see the correspondences.

Attention! As you certainly know 3D printing does not necessarily produce exactly the theoretical dimensions of the model. To accommodate my printer (and the general trend) those models are 0.3mm wider than the theoretical values.
You should check the dimensions with a caliper and scale the model accordingly before using this to buy a ring.

The radial marks on the top surface of the ring are meant to help you measure the size precisely along a diameter.