On what date did Atco Records release the album where Pantera introduced a groove metal sound?
Pantera:  With its fifth album, 1990's "Cowboys from Hell", Pantera introduced a groove metal sound. Cowboys from Hell: Cowboys from Hell is the fifth studio album and major label debut by American heavy metal band Pantera, released on July 24, 1990 by Atco Records.
July 24, 1990