I can't help much on what you have, and I don't want to rain on your parade. However, this could be some Chinese import that no repair parts were available for even back when. Perhaps on of the experienced plumbers such as HJ, Terry, or Redwood can give a more definitive answer. One thing is for sure, this is a water guzzler 3.5 gpf toilet. It may be that a new toilet is in your future. If so, please do yourself a favor and consider a Toto. You can cut your water consumption by almost 2/3 and have a state of the art toilet with the occasional replacement part available in every hardware store at a reasonable price.