According to one of my books, (Birds of New Guinea: Beehler, Pratt and Zimmerman) G.s.sclateri has maroon shoulders (coverts) whereas G.s.scheepmakeri has grey.
The only two Crowned Pigeons on show at Cotswold today are both lacking the white tips (they were bobbing about at the back of the exhibit so this is not a great shot but it does show both birds' 'crowns').
