Hi Roger, the original LP did credit Stephanie on the inner sleeve (along with several others from other labels) though not on the back cover or label.
Track #5 is actually a duet with Stephanie Mills, although there is no mention of that in the credits on my copy, or on this 1987 PIR Release, though I notice that the 1993 Right Stuff reissue has been entered on 45Worlds with Miss Mills being credited.
In 1980, when the original LP was released, Stephanie Mills was signed to 20th Century so presumably Philadelphia International weren't allowed to credit her for contractual reasons.