I'm no lawyer, but I write contracts almost daily. Reads to me like you acknowledged an agreement. If contested legally, you'll have to prove that the additional pages were not presented to you prior to your signature. Then explain why you signed it without full prior review.

That being said, your former employer would look awfully bad in court if shown to have been less than honest when asking for your signature on a document. But, its your responcibility to read before signing.

Good luck on this.