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It's the standard procedure for when customs stop something that doesn't need a permit.

From the letter customs send out:
If these goods are intended for an A category weapon, then consult your...

free-standing grip, in relation to a firearm, means a grip that,—
(a)is designed to be gripped by the whole or most of the trigger hand of a person firing the firearm;

You get your AO to confirm it with them by email.

Not on a pump action.

There isn't one. You can use 100 round drum mags if you want.

There are plenty of better reasons to apply for an E cat. It's not hard to get.