Provided further that where the capital asset became the property of the assessee before the 1st day of January 1939, he may, on proof of the fair market value thereof on the said date to the satisfaction of the, Income tax Officer, substitute for the actual cost such fair market value which shall be deemed to be the actual cost to him of the asset, and which shall be reduced by the amount of depreciation, if any, allowed to the assessee after the said date and increased or diminished, as the case may be, by any adjustment made under clause (vii) of sub section (2) of section 10;" Computation of the capital gains under section 12B is to be made by deducting from the market value of the consideration of the sale, exchange or transfer, expenditure incurred in connection with such sale, exchange or transfer and the actual cost to the assessee of the capital asset or at his option, where the capital asset became the property of the assessee before January 1, 1939, the fair market value of the asset on January 1, 1939.