HRRs are introduced by Tony Plate in this paper: Holographic Reduced Representations as well as this book, also by Plate: Holographic Reduced Representation: Distributed Representation for Cognitive Structures
HRRs are also used in this paper: Associative Long Short-Term Memory. (This is also the way I found them).
I’ll admit that right now I don’t understand HRRs at all. Hopefully this will get better over time.