IOS16 was an IOS, installed and used exclusivley by the Wii Backup Disc. When Nintendo went to fix the signing bug in all versions of IOS, they didn't think to release an updated version of IOS16, since it was never publicly distributed.
After that update, IOS16 was left as the one last IOS with the unfixed signing bug. It was then unofficially released and used to downgrade the System Menu prior to the 4.0 update, primarily to assist in piracy.
|Version||Signing bug?||STM Release Exploit?||Notes|
|v257||Yes||Yes||Installed by the Wii Backup Disc|
|v512||N/A||N/A||Stub IOS released alongside System Menu 4.0|