Yes thank you for your response. I have wondered if putting my shotgun in its leather takedown case counts as disguising it as a piece of luggage. I would imagine the test might be can the thing be fired when in its walking aid/crutch format or only fired when it is assembled.
However a different test might be what was the purpose of the crutch format was it to enable the firearm to avoid it being noticed as such? Whereas a gunslip or leather takedown guncase isn't. OTOH I have handled once upon a time the famous H & K briefcase.
I think it is this "concealability" fetish that beset Michael Howard when the pistol ban was enacted and the creation of the size restrictions you mention. I think if we did have one we'd all want better than a 4x20 BSA air rifle 'scope on it!