Real clear what is going on here.
It’s a continuation cord. You see, the wiring works like this: The receptacle is split on both sides. The incoming power is double lugged on the bottom outlet. The outgoing power (to continue the circuit) is double lugged on the top outlet. You have to use the continuation cord to get the power from the bottom outlet to the top one.
Like I said, simple! /s
I started on a Pr1me 550 type II learning BASIC myself. Apple ][s came out about 4 years later. Then I used them. Windows SA now.