It's a nice photo and I know they listed it as Brass' Friarbird when they had it, but it's a Little Friarbird. Perhaps they used the only book that covered this species and was easily available at that time (Birds of New Guinea from 1986). It has a decent illuistration of Brass', but a rather strange-looking Little Friarbird that doesn't look much like the actual bird, so they probably went with the next best thing. Regardless of the reason for the Heppenheim's mistake (which is repeated on zootierliste and ibc.lynxeds), the bare skin around the eyes of Brass' is less extensive and all dark, and its upper chest is much darker resulting in more contrasting white streaking.
Thanks a lot for this absolutely correct remark! At that time I trusted the sign and there was hardly literature to cross-check available. I also corrected it on Zootierliste and will so on IBC.
