There is a fair amount of info on Army Navy Cooperative Society in Nigel Brown's books. (including serial number info)

You might want to check there. Dramatic differences in quality exist between guns. If you can find a best quality gun the lack of appeal of ANCS will make it quite a bargain compared to its worth if the actual maker's name was applied to the gun.

I believe Webley, Osborne and Rogers were suppliers to ANCS pre-1899 but this is only going back to foggy memory from things once read.