See this post for more background: Ducati ST3 front sprocket
I had another ST3 in this week with the same issue, so took some more photos.
Spacers: ST3 (999, etc) on right, 1000SS on left. 19.7mm vs 14.2mm (or something like that, forgotten already).
Sprocket fitted with incorrect spacer.
Sprocket fitted with correct spacer.