Form 1040 (1864) [pdf] (irs.gov)

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.

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.

We really had a flat tax rate then? Those mean the poorer pay the majority of the total taxes.