205. The property of the Federation shall, save in so far as any Federal law may otherwise provide, be from all taxes imposed by, or by any authority within, a unit:

Provided that, until any Federal law otherwise provides, any property of the Federation which was immediately before the commencement of this Constitution liable, treated as liable, to any such tax shall, so long as that tar continues, continue to be liable, or to be treated as liable, thereto.