I remember seeing Rock'n'Rye often, but I rarely look at the offerings in that part of the liquor store
. I promise to research more thoroughly on my next sojourn to Party Source.
Ways to make whiskey more palatable to people who don't care for the taste of straight whiskey seem to go to either extreme. Adding bitters or wormwood (vermouth) is one direction; the other is adding sweetners (honey, fruit juice). Some additives (orange, for example) use both ideas. Hence, Southern Comfort (yes, I know there's no whiskey in SC; but most drinkers of it don't). Rock'n'Rye is one of the sweet ones.
Many of us learned to make rock candy when we were kids. You made up a sugar syrup in a jar, then suspended a string with a weight on it into the syrup through a hole in the lid. As you watched (over a period of days; not continuously, of course) sugar magically crystallized on the string, getting larger and larger. It was a great treat, and was also something that kids have been doing since at least colonial days.
The distinguishing feature of the R'n'R I remember is that the rock candy was grown in the bottle before filling, so that it was larger than the neck of the bottle. This lent a sort of "ship in a bottle" curiosity to the whole thing. There is a type of pear brandy made in France where they actually place the empty bottles on the pear buds in the spring and allow the fruit to grow inside the bottle. It does nothing for the flavor, but the "how-did-they-get-that-in-there" effect sells a lot of brandy.
I guess, for those who understood these things, the rock candy was also a good indicator of proof. Since the sugar crystals don't dissolve in alcohol their presence would indicate the contents weren't too watered down.