Air Compressor Repair - What have I done!

I traded this air compressor with another one that I had and I can't say if it was a mistake or a blessing. At the end of it all, I learned few things along the line.
Parts list below:
Pressure Switch with Pressure regulator: https://amzn.to/43W8kF4
Rectorseal: https://amzn.to/45g9tYT