I recnetly Googled the search term 'ico and icns' into Google. I was merely trying to get an idea of the popularity of each format on each OS. Anyway, on a website I see someone wrote about another Mac OSX icon format extension .RSRC
I managed to find this website which had a link to an actual RSRC file (picture above). Here is that download link: http://hijinksinc.com/downloads/iTunes10.rsrc.zip
or http://www.sendspace.com/file/4nafph
A discussion relating to viewing the format on Windows: Here
What do you think, can it work? Does it have specific pages i.e 16x - 256x +
History
Created by programmer Bruce Horn, resource fork (.rsrc) files are a type of virtual memory originally created for use in the pre-Macintosh Apple Lisa in 1978.
Later Refinements
In 1985, Apple implemented resource fork files for the first time in a Macintosh model. The Macintosh XL used more advanced .rsrc files to reduce the memory consumption of the Mac XL down to 128 kilobytes.
How They Work
A file stores data in its data fork (such as .jpg) but stores other file information, such as its embedded icon graphic, in a resource fork. Most .rsrc files are automatically hidden from the user's view.
Opening .RSRC File
You can locate and open .rsrc files with Apple's ResEdit software in Mac OS 9 or earlier versions, but in OS X you need to use Apple's Terminal application to view or modify an .rsrc file by appending "/..namedfork/rsrc" to your command.
RSRC Files and Windows
Because .rsrc files are limited to a hard-drive format that Windows doesn't support, there currently is no way to open an .rsrc file in Windows.
Warning
Rsrc is a developer file that shouldn't be opened by most users. Changing the structure of the resource fork can cause file data loss and recovery problems.
I've also uploaded \XnView.app\Contents\Resources\appIcon.icns from XnView-macosx.zip to iConvert (Note: Options » Convert to rsrc icons is turned off by default): appIcon.rsrc
