Reference numbers indicate a watch's specific model and/or model variant and can be composed of letters and numbers. Some manufacturers put the reference number on the case back (not to be confused with the case number), between the lugs, or inscribe it on the dial. You should always be able to find the reference number in your watch's documents and on the invoice. If you cannot locate the reference number, search for the manufacturer (often found on the dial) and compare the image results with your watch. This quick bit of research should help you in most cases.