Installing used Culligan water softener + Potassium tank

I just acquired a used Culligan water softener with a brine tank and another tank which I assume to be a green sand filter since it has a lid on the top for adding something (Potassium?). Manuals that came with describe the systema s a Culligan Mark 89 or Mark 812. Previous owner says they are about 10 years old. Not much information on the potassium tank. Model number on the water softener head is 445603, I cannot read the model number on the potassium tank head. Our water is not hard and has a little iron but not much according to our well driller. Problem is it stinks of Hydrogen Sulfide and we have Bacterial Iron.

I called the local Culligan dealer for some advice but it became obvious they weren't going to help me with any information since I was not waving any money under their noses.

Questions are:
Is this actually a green sand filter?
How is the potassium tank plumbed in related to the water softener? Before softener or after it?
How or what is normally added to the "potassium" tank and how often? Dump in granular potassium or mix it wit water and add it? Strength?

The normal set up is the GreenSand/pot per unit first and then the softener.

The Mark 812 most likely would be the pot perm and the Mark 89 is the softener , but that is my best guess.