The tin in the case is merely a push on top tin and would rot away in the soil as fast as a bean can. It is a Japanned tin with shellac on it and normally holds percussion caps and occasionally whale grease for greasing the bullets so all chambers of the gun dont go off at once - which could be disastrous.
Your top is a new one to me and certainly is not part of an old pistol or gun cased accessory to the b est of my knowledge and I have seen a few of them in my life. It looks as though it may be a pewter top for something but what I do not know.