(ii) where there is more than one deficiency and each such deficiency relates to a different assessment year, the deficiency which relates to an earlier assessment year shall be set off under this sub- section before setting off the deficiency in relation to a later assessment year: Provided further that in the case of an assessee being a co- operative society, the provisions of this. sub- section shall have effect as if for the words" fourth assessment year",- the words" sixth assessment year" had been substituted.