For the purposes of this Part of this Act, any contribution made by the Crown in aid of rates in respect of any premises occupied by or on behalf of the Crown for public purposes shall be treated as money paid as rates, and, in the case of any premises which, if in rateable occupation, would be agricultural, industrial or freight transport hereditaments, the value upon which that contribution would if this Act had not been passed have been computed for the half-year beginning on the first day of October, nineteen hundred and twenty-nine, shall be treated as the unreduced rateable value of the premises and the value on which that contribution is computed for that half-year shall be treated as the reduced rateable value thereof.