Just as a postscript to this thread, Jose Fernando Pacheco, coordinator of the Brazilian Records Committee tells me that Hellmayr (1910. The birds of the rio Madeira) mentions "iris brown" for the Plumbeous Pigeon Columba plumbea pallescens collected at Borba. Unfortunately, Hellmayr didn't collect any specimens of Ruddy Pigeon in the area.
Pacheco suggests that iris colour may be a valid diagnostic indicator only in certain parts of the respective ranges.
So I think I'll leave the id as Columba sp. which, following AOU, should now be Patagioenas sp.