The Ilford XP-1 *film* is the predecessor to XP-2. i.e. it's a black+white chromogenic film intended for process C41. But I suppose you already knew that.(posted 8918 days ago)XP1 was the first of the mono C41 films, and although it gave good results in C41, Ilford recommended their own special developer for *best* results. XP1 film was replaced by XP2 when Ilford improved it so that it didn't need special developer any more.
So what you have is a developer intended for film that has been obsolete for a decade or more.
A photo book from the mid 80s might touch on the characteristics of XP1 developer. I recall reading the above info in one of my old books (I have an extensive collection gained from charity booksales and library disposals).