Nothing herein contained shall require any change in the plans, construction or intended use of the building, for which a building permit has been issued prior to the effective date of the Zoning Ordinance and the construction of which shall have been diligently prosecuted within the six months of the date of such permit, and the ground story framework of which, including the second tier of beams, shall have been completed according to such plans as filed within two years from the date of the passage of the Zoning Ordinance. Nothing herein contained shall prevent the restoration of a wall declared unsafe by the Mayor. (Ord. 545-47. Passed 10-20-47.)