Differences between Polk Monitor 7?

Just picked up a pair of Polk Monitor 7 speakers at a yard sale. Anyone know the difference between this version and the models indicated as Monitor 7A, 7B, 7C? For some reason there seems to be a fairly significant difference in the Blue Book value between these variations with the original versions (Monitor 7) listing at a greater average used price then what appear to be later (upgraded?) versions of this model. Anyone know the differences? Thanks.
The main difference was the tweeters used IMO. The original Monitor 7A and early 7B used a Peerless tweeter and the later models used a Polk tweeter. The Peerless is considered to be superior by many. Polk Audio has an extensive forum on their website that will answer most of your questions. One of my first systems aquired new in '78 was a pair of 7Bs and a NAD 3020. It was a really nice system that I still have in storage.