U or not a u

2008 W.rev of 2007 Silver Eagle, I was just wondering, when was the last time something like this was done to the reverse of a coin? or to change a Letter from upper to lower case? After Looking at just some of my coins and looking through the red book, I cant find any coin with the Lower case `u` in United? Thats Big,if you find 2008W with the upper case letter its the error coin? and the lower case is the change to the Silver Eagles:}? O.K. Im confused. You gotta Love this hobby, I do.