What is the test for a final judgment on the merits of an amended complaint?

"Under res judicata, a final judgment on the merits bars further claims by parties or their privies based on the same cause of action." Montana v. United States, 440 U.S. 147, 153, 99 S.Ct. 970, 973, 59 L.Ed.2d 210 (1979). Our comparison of the claims made in the second amended complaint in 82-463 and in the complaint in 83-329 leads us to agree with the conclusion of the trial judge hearing 83-329 that the causes of action in each complaint were substantially the same. The more difficult question in this case is whether denial of the motion to amend constitutes a final judgment on the merits of Marin's appeal from the decision of the P.R.R.B. We hold that under the unusual circumstances of this case it does.