The tax code in 1940 was not the current code with different rates but a different code entirely. To make an example (I am not a CPA and even less so a 1940s CPA): if you could have deducted your living expenses (food, rent/mortgage, clothes, vehicle etc.) even at the current rate you would not have been paying nearly as much in taxes as somebody who has to obey the current tax code. Same rate, different tax.

You really cannot express the tax regime with just a brackets table, the tax code alone, without IRS regulations and case law is 2600 pages as of now.