There is a model number located on the back, in very small print. It starts with "A", like A1234. If you have selected the correct item, then click the "Add to Cart" button. Also verify the strap size you need.
If you are unsure what size you need, try measuring your flexed bicep. We currently offer the following (5) sizes: 9", 12", 15", 19", and 22". Round up to determine the correct size, unless your arm size is very close to one we offer.