I would think that the extension by itself and not installed isn't actually a prohibited magazine... Because it's not a magazine. It's just a piece of threaded tube and can't do anything if it's not screwed on. By that reasoning any +2 or larger 870 extension is prohibited because it can also fit on an 1100. And any Nordic extension is prohib because they're universal and used with a firearm specific nut.
It's a tubular magazine. Welcome to Canada's ridiculous gun laws, lol.
By your logic, I could install a crazy long rem 870 mag tube on my versa max, because they have the same thread. And technically, I don't think there's any provision in the law that contradicts that idea. But yet somehow we know that we can't do that. ¯\_(ツ)_/¯




















































