5Where a company has issued shares or debentures to a person in exchange for shares or debentures in another company in a case falling within paragaph 20 of Schedule 3 to this Act, the amount of any development gain which accrued to him in respect of the disposal by him to the issuing company of the shares or debentures in the other company shall, in computing under Schedule 6 to the [1965 c. 25.] Finance Act 1965 the gain accruing on a disposal of the shares or debentures so issued to him, be treated as an amount allowable under paragraph 4(1)(b) of the said Schedule 6.