The magazines themselves are prohibited. It makes no difference if you own the matching weapon or not.
But, as I've posted somewhere (might be this thread), no single part of a mag legally constitutes a mag as the "frame or receiver" legally constitutes a firearm. All of a mag's components are, therefore, under the legislation, simply uncontrolled parts until they are assembled into a complete mag. That concept has not yet been tested in the courts, however - but to rule contrary to that would require a judge to decide which part, if any, does and that requires a legal leap. The "pinning" guidelines are exactly that: guidelines. They are not written into the legislation or its attendant regulations. "Suggestions" in the "guidelines" include pins through the body or a post upwards from the baseplate, hence the inability to legally determine which part must be modified. The courts are supposed to interpret legislation, and not write missing bits in.
One could, then, pin bodies on some mags and "permanently" (again, not defined) attach posts to the baseplates of others and, should one subsequently go "postal", swap the parts to provide some whole mags (and some ruined ones). One could also simply drill pins out, too.
As the late Dave Tomlinson of the NFA was fond of saying. "It doesn't have to make sense, it's government policy".