I remember doing this back in the 1960s after reading an article in one of the outdoor magazines. But I forget exactly how it was done. After a lot of searching I finally found a single thread in a fishing forum that discusses them although it didn’t seem like any of the posters had actually done it, they knew someone who had.
The method they mention is just to remove the lens from the fish eye and boil it in water for 10 seconds. The lens gets round and hardens with a very nice luster.
I recall the method I used was a bit more complex and included wrapping the hot lens in soft cloth for several hours. I thought it used a vinegar solution also.
They were quite remarkable when done. You do need a large fish to start with, I used carp. Has anyone else ever heard of this?
Dennis