Mustang Seats Backrest that was bought for my 2016 Road Glide. Only used a couple times, so it's like new. This backrest works with Mustang seats. Not sure what other models it fits, so check fitment for your bike.
Interested but would you know the model number by chance? I have the Super touring summit extended which apparently takes only one backrest.
The best I can determine looking at Mustang's website it is a 79557. Below is a screen shot from the page. I could reach out to Mustang, but I believe they are closed due to the lockdown.