I think there's a common misconception that numerical codes should be accessible without documentation, and without a background in the subject.

https://github.com/nasa/NASTRAN-95

Take NASTRAN-95, for example. It has good documentation in the form of manuals, books, and papers. If you have the mechanical background, you should be able to understand the docs explaining the implementation approach, and then you should be able to understand the code. It doesn't matter that there are no comments anywhere. Aside from some cosmetic differences, it looks pretty much like what you'd write today with MATLAB. It's perfectly readable and accessible by the target audience.