747 classic originally used Depleted Uranium in the flying controls for balance. It could be found in the elevators, rudders and outbd ailerons. It was also originally fitted in the early JT9d nose cowls (different amounts depending whether the cowl was for an inb'd or outb'd position) again to reduce flutter.
The maintenance practice for it was to ensure the paint was intact and they weren't corroded.
JFI we had a visit from the NRPB sometime after the loss of PA103 as they were interested on roughly how much there could have been fitted.
Made a damn fine door stop or rivet block though
