It is surely unsafe to assume he would have known something when he
apparently does not know it now.
Quite correct. The burden of proof in criminal law is that it must be proven beyond all reasonable doubt, and the fact that he was in possession of a weapon, would usually suffice for this. Then it would appear that defending council felt they would be unable to succesfully contest this, and therefore advised a guilty plea.
It's rather unfair passing the buck onto the arresting officer, for not using some discretion. It's a bit difficult offering words of advice, when the house has been raided and searched by a number of officers. We are not talking about a child having his catapult confiscated!