Yes, it may be legal for quite some time, if it is pinned to five rounds.
Then, one day, the RCMP or local cops will bust someone who has one of these mags pinned to five. A Crown prosecutor will make the argument in front of a lower court judge that the law regarding magazine capacity is meant to protect the public against the menace of terrorism or a crazed gunmen. An RCMP weapons expert will testify that the magazine is “easily converted” into an illegal large capacity magazine, merely by removing the pin. The magazine then becomes an instrument of terrorism and murder. The cops will coordinate this all with press releases showing the magazine being used by a machinegun.
The judge will rule in favor of the Crown and declare that this, and all other “easily converted large capacity magazines” are outlawed. And if the lower court does not go down this path, the Crown will appeal to a higher court, gaining the same result from an entirely predictable judicial system. Crown directives and, if need be, regulations will be suitably modified to identify all pinned magazines as prohibited devices. An amnesty will be announced for all "law-abiding citizens" to turn in their magazines for destruction.
In my view, it is not a question of if it will happen, but when.
Big