Form 1040 (1864) [pdf]
irs.gov
3 threads
Looks like you can find this page by going to
https://www.irs.gov/prior-year-forms-and-instructions
Then sort by "revision date". 1864 seemed to be an outlier among that list, everything else is 1913 or later.
I would love to see a rendered timeline of the history of these forums and a year by year highlighted diff. It would be cool to see what the explanation was. I'm sure there were many interesting reasons for additional fields that we don't know of today.
We really had a flat tax rate then? Those mean the poorer pay the majority of the total taxes.
There was a $600 deduction, so common laborers didn't pay any income tax.