I don't think it makes any difference whether it is a -G or -C; That is why this clause was in the contract. I think the US would approve any approach by the Australian government - but the Australian government is very unlikely to use any of its political capital with the USA on this; as far as politicians are concerned it is simply unimportant.
Asbestos risk is simply an excuse. It applies to virtually anything of that vintage! And would be pretty minor anyway, compared to most WW2 aircraft/vehicles.
John

