The Society changed its view about Paul and Matthias in 1971. There was a Question from Readers on this in the May 15th issue.
What's really funny is to see the differences on this in the old Aid book. The first part of the Aid book had been published in 1969 and in the article "Apostle" they said Paul was one of the 12. The second part was published in 1971 and the article on Paul said Matthias was one of the 12, not Paul. The Insight article on Apostle was re-worked to fix this.
If you don't have an Aid book you can read it here:
http://www.reexamine.info/70s/AID.pdf
It's a LONG download, however.