There are a couple of ring size sets; the advantages of this one:

• size and shape of typical rings, rounded inside
• no tabs etc, so you can wear a ring for hours to check if it feels comfortable
• each ring has it's ring size imprinted in 4 ways: diameter (d), European size (E), North American size (A) and UK size
• 4 sets available, so you get round values in your preferred system:
  • diameters from 15mm to 22.5mm in steps on 0.5mm (16 pcs)
  • European sizes from 50 to 69 (20 pcs)
  • North American sizes from 4 to 13.5 in steps of 0.5 (½) (20 pcs)
  • UK sizes from G to Z (20 pcs)
• Embossed version (as in photo) or multicolor version (rendered image) available.

Disadvantages: They need more storage space than thin sets and there's no nice storage option. I just keep them on a piece of thin rope, closed by a knot.

Notes:

• The model has exactly the noted inner diameter, but if printing extrudes a bit too much, the resulting diameter gets a bit smaller. I first measured about 0.15mm less than intended, but just slightly scratching the inside with the caliper itself brought it to pretty much the exact diameter. I recommend checking the actual diameter with a caliper, and if too small, using a bit of sand paper.
• The UK sizes seem to vary a bit within various online resources. I used the British Standard 6820:1987 (BSI).