This reads so much like an urban legend, that I had to poke around a bit. It appears that it was a piece of fiction written by a Williston Fisk for Harper's Weekly in 1898, and has been given various backstories as time went on.

here's a poem by ryokan expressing a similar sentiment

My legacy—What will it be?

Flowers in spring,

The cuckoo in summer,

And the crimson maples

Of autumn...

This were the writing skills of a random dude who was stuck in an asylum. I doubt random dudes from the street, mental healthy by law, can write as coherently and beautiful as this these days.

>I, Charles Lounsberry, being of sound and disposing mind and memory...

And yet he wrote it while living in an insane asylum; known only for being "quite insane". The exact opposite of having a sound mind.

https://www.law.cornell.edu/wex/disposing_mind_and_memory

>"Most Beautiful Will Ever Made"

Not sure about "most" part but beautiful it absolutely is.