The letter stated "to determine admissibility" If the stock is the same item as bought off the shelf in Canada - talk to the dealer/importer and get the commodity code for his stock. Send a response to customs quoting the commodity code and that the item is routinely imported lagally into Canada. You'll get it. If they try to refuse it appeal. Since it falls into a category of imports for which prohibitions exist, It was grabbed likely because it did not come with a proper declaration and its being held to determine if it is a prohibited item. This is routine, actually, and frequently a result of improper import papers. It pays to do your homework when importing legal goods (where prohibitions exist) and give your shipper the commodity codes for your product, for his declaration.
For those of you who don't know how customs deals with these one off imports - it goes like this: the parcel arrives at a customs entry point, ether within a postal sorting facility, bond shed or customs long room. If there is documentation attached, its reviewed to see what the item is. If the officer is satisfied that it is what it is, he codes up an E14 form and away it goes.
If it doesn't come with paperwork, he examines it, sits down with his "harmonized code" (the 10-digit "classification number") to see where it belongs. If the code group contains prohibited items, he then has to determine if the item he has fits this, or not. If he does not feel technically competent to judge the item, he sends it over for an admissibility review (in the case of the ATI stock, he might not know whether the ATI stock fits the description of a "bullpup", because the commodity description is general enough that, to him, it might). Before you jump on customs officers that he should "know", remember, that in print form the code and tariff is several inches thick, and the descriptions general. Virtually EVERYTHING has a commodity code, and many items can be classed under different codes. With the heightened security issues at the border, an officer is not going to take a chance on pooching his career because he was the one who let the prohibited item through that hung the Minister out to dry.