This is a pretty modern medallion, John. It was sadly not made in 1789, but commemorates the famous 'Mutiny on the Bounty' on April 28th of that year. It was produced as part of a set sometime in the 1970s, probably in the United States. It should weigh 13 grams and measure 30 mm in diameter.
William Bligh was the captain of the HMS Bounty ship, and he was overthrown by the crew under the leadership of Fletcher Christian, who was later killed in an argument with Tahitian natives.
When these medals look nice like in our picture, they seem to have a bit of value - approximately $15 US dollars. With wear and scratches the value drops severely.