I have a Koyo sewing machine. I do not know the model or have a manual. It's missing an 'o- ring'. Where can I view the old models of Koyo sewing machines?

You could try contacting a dealer that specializes in old or antique sewing machine parts, such as this one. At least you could ask them for help in locating the information you need. You could also put the key words 'koyo sewing machine old parts' and explore the results page from that - there are several antique specialists who may be able to help.