Section 42. Any society may, in the annual statement required by section forty-one, value its bonds or other evidences of debt having a fixed term and rate and not in default as to principal or interest and if amply secured, in accordance with the following rule:— if purchased at par, at the par value; if purchased above or below par, on the basis of the purchase price adjusted so as to bring the value to par at maturity and so as to yield meantime the effective rate of interest at which the purchase was made; provided that the purchase price shall in no case be taken at a higher figure than the actual market value at the time of purchase and provided, further, that the commissioner shall have full discretion in determining the method of calculating values according to the foregoing rule; and provided, also, that any society may return such bonds or other evidences of debt at either their market or their book value but in no event at any aggregate value exceeding the aggregate of the values calculated according to the foregoing rule.