Magazine spring, plug, and retainer installation is explained in the Owner's Manual. The retainer isn't essential, it's a safety device to prevent injury from the spring letting loose when people don't expect it. However don't trust it, the retainer has been known to unexpectedly slip loose. Rarely, but you don't want to be the guy that finds the exception with his head in the line of fire.

The key elements in human thinking are not numbers but labels of fuzzy sets. -- L. Zadeh

Which explains a lot.