Complex sentence: Each manufacturer used its own unique set of disc sizes (varying depending on the exact model purchased) thus assuring that once the purchaser had bought his music box, he had to buy the music discs from the same manufacturer.
Split into simpler sentences: Each manufacturer used its own unique set of disc sizes (which varied depending on the exact model purchased). This assured that once the purchaser had bought his music box, he had to buy the music discs from the same manufacturer.