If you don't install the bars, then your emergency/parking brake will not work.
Well, this is ironic. I went to look at a shop manual for an early car and all I could were parts books.
Earlier today I wanted a 51 Parts book and all I could find were shop manuals.
OK, there is a lever that mounts on the rear shoe on the rear brakes. The emergency/parking brake cable attaches to the bottom of this lever. The 'mysterious' bar fits between this lever and the front shoe just under the wheel cylinder. You pull on the cable and the lever pushes the bar against the front shoe and forces the front shoe against the brake drum. If you examine the bar, you will find the end that goes against the rear shoe & lever has a slot wider than the end that goes against the front shoe.
Yes, the emergency/parking brake only engages the front shoe and that is why it is more effective at holding the car from going forward than it is when backing up.