Back in 82 and 83 I spent some time riding with my brother on a wrecker. Too long ago to tell you chain size. The chain in the center is there to keep the sling from swinging around when not in use. Every driver did this differently. That big hook ( that you have circled) is a J hook you use to hook up the car. Does the same job to hook the sling as a smaller chain. You use what is on the back if the truck most of the time. Around here you would leave them on the back of the truck between tows, unless you went into a sketchy place.