Orca, our bombing circles had full size sand replicas of aircraft too. You must have come after the tide went out
Seriously, I went out to the old Wainfleet #1 up near Gib Point. There was some plastic covered chain link lying in the sand and the bullseye was a smooth semi-specifical green painted object as I would imagine a T54/55 turret. The metal however was millimetres thick and thinner than any vehicle except a truck cab but not that shape.
We didn't have a shovel to explore further.