According to Aviation Week, this was a secret Magnum (satellite) ELINT (ELectronic INTtelligence) gathering satellite headed for geosynchronous orbit like those launched by STS-51-C and STS-33.
According to Aviation Week, STS-38 launched a secret, Magnum (satellite) ELINT (ELectronic INTtelligence) gathering satellite into geosynchronous orbit. Identical satellites were also launched by STS-51-C and STS-33.