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.
Rick
Rick