Yep, I know where they go. 4Xoverland has them on his troopy, or something similar. I ride with my arm there anyway and can always use another cup holder.
I saw these on an aussie youtube channel and looked them up. They are for the 70 series but the site says that with a mod to the bracket it can work in 80s, 100s etc.
Just ordered a set for my HDJ81, so wondering if anyone has done the mod already to use them on an 80...