Okay, the "spring rate" signifies the extra weight options add or subtract from the normal weight of the car. For instance, you'll notice in the chart below (from the September, 1968 P&A 34) that some options add weight while others reduce weight. In your case, 84 equals 84 pounds more than the normal rear weight of the car. The same goes for the front. In your case, your options added an additional 259 pounds of weight to the front of the car. Now, because of this added weight, the "normal" springs would have to be replaced with others so the ride height would remain the same. This is how the factory determined the spring rates to keep the ride height at the normal level.
The F41 option is actually "Special Purpose, Front and Rear Springs" which gave the car a bit more harsher ride compared to the "Standard" springs. The G31 option was "Heavy Duty REAR Springs" only, and I believe was used to eliminate wheel hop off the line. This was a racing options and very few street cars came this way, unless of course, the owner wanted to go racing!!
The Spring Codes like "BL" and "BM" were used at the factory to identify the spring. Basically it kept the line workers from having to go search for the correct spring. All major assemblies had some kind of code either stamped into the part, cast into the part or had a sticker affixed to the part to help the line workers quickly identify what they needed so they didn't waste time searching!
By the way, sorry for the "bleed through", but the pages in the P&A aren't that thick!!
So anyway, does this help?