How much is Solar Radiation effected by Axial Tilt or Obliquity?
Check out this matlab program that is presented in a paper by Kostadinov and Gilb, 2014. The paper is worth reading, and they have their full matlab code in the "supplement" link (top right corner under downloads). 

The code launches a GUI. From it you can control the values of obliquity, eccentricity, precession, total solar irradiance, day of year, calendar start year and end year,  and latitude. Section 2.1 in the paper explains the coordinate system. 

I don't think anyone could answer off the top of their heads the question "what is the difference between 1920, 1939, etc... of solar radiation at a particular geo-location". But with this program, you can mess around with the data and find out. So let us know what you discover! (ps, if you don't have Matlab, you can probably use this in octave which is free). 

Here's a screenshot of the gui


And here's what some of the output looks like 



Here's some alternatives to matlab: 
1) nasa has online calculator that spits out raw text tables of insolation.
2) If you are savvy with fortran then you may like this fortran code.
3) The fortran code above also has an online interface
4) If you like python, then you may like this python code, climlab, where there are detailed instructions here on how to use the code and visualize the information.
5) If you like R, then try palinsol which has a little documentation here.