The type of swivel you need will be determined by the type of tongue and coupler on your trailer. Many trailers use one of three popular coupler brands, Fulton, Shelby, or Dutton-Lainson. However, there are literally dozens of knock-off brand couplers that have no name. In many cases, their measures correspond with one of the three major brands.
If you'd like to install a swivel on your trailer with a coupler that has no name, you'll need to take some measurements. I've created a simple form you can use to take all the measures you'll need to order just the right swivel. Take the measures and e-mail or fax the completed form to me and I can help you find the right swivel for your application. If a custom order is necessary, I can often work with the manufacturer to make that happen for you.
Here's a link to the form: Swivel Hitch Adaptor Measurements
Safe riding and happy swiveling!