Well, the rings are all drive bands from 20mm projectiles. The two rusty objects on the left appear to be spent 20mm projectiles and also hollow, which is lucky for you ! If they weren't hollow, you could very well have taken home the equivalent of two small but quite lethal grenades. If you find any complete ones of these, PLEASE leave them alone. High explosive (HE) 20mm projectiles are notoriously bad tempered.
As for the brass item on the bottom, this is the top cover from a 2inch mortar nose fuze. I have attached a picture of a sectioned fuze for you to see. Just to explain the fuze 'action', when the bomb is fired, the force exerted on the rod and spring to the right forces the rod down. This allows the ball to travel down the little slope and thus 'arms' the bomb. The firing pin is now forced against the inside of the brass cover under action of its spring and, when the bomb hits an ojbect, the brass cap is crushed against the firing pin which hence detonates the bomb. The cap you found clearly shows the impression left by the top of the firing pin.
Be careful on the site as you will surely find live ordnance. If you find a complete mortar bomb with the fuze attached, DO NOT MOVE IT UNDER ANY CIRCUMSTANCE$. Call the authorities straight away.
2 inch fuze.jpg
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