On a recent Wollongong pelagic (June 2011) a Silver (type) Gull with 2 mirrors on p9 and p10 (but no third mirror on p8) was photographed. Photos have been posted at http://www.tonykeenebirds.co.uk/rand...ollongong.html
It has been suggested on birding-Aus that it might be a Red-billed Gull C. n. scopulinus from NZ or a Silver Gull from WA. I don't think it is either. I think it is a Silver Gull from within the normal, large and poorly known range of variation for Silver gull in south east Aust. I posted my reasons to birding-aus but there have been no responses yet. I have posted photos of two spread wings of Red-billed Gulls from the Museum of NZ (Te Papa Tongerewa). They show a perpendicular (of sorts) demarcation at the basal edge of the mirrors. The one of the upper wing view actually has 3 mirrors, and note that p10 is not fully grown.
Also I have posted two old photos of adult Silver Gulls from Hobart for comparison. They are almost 2-mirrored. Compare the shapes of the basal demarcation of the mirrors. This seems to be the best distinction between Silver and Red-billed. However, there is quite likely to be a lot of variation and perhaps considerable overlap in this character. The whole subject is poorly known and these thoughts are far from definitive.