You're essentially correct.
You'll find that your mystery spring is only on the rears. That's because it's a tension/anti-rattle spring on the strut that goes to the parking brake lever. It's not in your kit. The 4 springs are brake shoe holddown springs. I reused mine when I did a rebuild.
Here's a better pic of a 51 GMC (Bendix brakes just like yours) that shows a good view of the spring in place.
http://s456.photobucket.com/user/bl...ogress/DSC_1244.jpg.html?sort=3&o=10David
