§ "13. That as from the nineteenth day of April, nineteen hundred and twenty, the ad valorem stamp duty charged on statements of the nominal capital of any corporation or company, and on statements of the increase of any such capital, and on statements of the amounts contributed by a limited partner to a limited partnership, and on statements of any increase in any such amounts, shall be at the rate of one pound instead of five shillings for every £100 or fraction of £100."