Shotgun magazine plugs are acceptable for hunting regulations, not for the Firearms Act.
Barrel extensions were acceptable, but no longer are. This is simply because the folks who make up the rules said so. They changed their minds. This was not a matter of OIC, legislation, it was simply a matter of interpretation over what determines the length of a barrel. You don't have to understand it. But barrel length is the sole reason that the Marlin Camp Carbine, the Ruger PC9, the Beretta, etc. are restricted. Rebarrel them, and they become non-restricted.