The stockholders of every bank shall be individually liable, actually and ratably, and not for one another, for the benefit of the depositors in said bank at the amount of their stock at the par value thereof, and in addition to said stock. However, persons holding stock as executors, administrators, guardians or trustees shall not be personally liable as stockholders, but the assets and funds in their hands constituting the trust shall be liable to the same extent as the testator, intestate, ward, or person interested in such trust fund would be, if living or competent to act. Persons holding stock as collateral security shall not be personally liable as stockholders, but the person pledging such stock shall be deemed the stockholder and liable under this section. Such double liability may be enforced in a suit at law or in equity by the receiver of any bank in process of liquidation. Such suit, however, shall be brought within six years from the date the bank went into liquidation and not thereafter. Such double liability shall not apply, however, to stock in any bank organized, after April 2, 1934, nor to stock in any bank open for business on April 2, 1934, provided such bank is a member of the Federal Deposit Insurance Corporation, or any other similar agency created by the laws of the United States.