The very rare to find Sigma 300-800mm Four Thirds mount lens is listed on eBay (Click here). The item has been spotted by our reader Peter who also wrote:
The “Sigmonster” is the longest and most expensive lens with a native FT mount ever made. Your readers can check the entire FT lens catalog at https://www.apotelyt.com/photo-lens/four-thirds-catalog , and can find an adapter to use the Sigmonster on an mFT-camera at https://www.apotelyt.com/photo-accessory/four-thirds-adapter . A decent size image of the lens is available at https://www.apotelyt.com/i1/sigma-300-800mm-960×640.jpg.