What is a "Key" to most collectors may not be the same ones others recognize. For example, the transition cents of the LMC series are perhaps the rarest ! The 92 CAM has only been found one time I believe, while there have been a couple dozen of the 92 D CAM authenticated. The 88 reverse 89 is abundant in comparison to the D, which is another extremely rare variety. Now a serious collector who want a complete set of LMC would certainly seek the rarest of them, including the 58 D DDO. But because of their extreme rarity, few people will ever obtain them. I seriously doubt if a full LMC set with transitions will ever be held by an individual. The coins needed just aren't for sale. And some that are are very expensive. I believe a 1992 CAM just sold for more than $12,000. The 92 D cams are in the thousands and no one knows how much the 88 D reverse 89 would bring. And this is a modern set. But each year the number of cent collections grows. Most limit to obtainable coins, but a few get the "bug" and really want an entire, complete collection. That keeps demand for the rarest ones higher and higher over time. For every thousand (I am guessing of course) that limit their collections, there is someone who is determined to complete theirs. IMHO gary