Notice under this contract shall be deemed to have been sufficiently given to and received by the Contractor when mailed in a sealed, post-paid wrapper and addressed to the Contractor. Whenever notice shall be given to the Contracting Officer, same shall be sufficient when mailed in a sealed, post-paid wrapper, addressed to the Chief Signal Officer of the United States Army, Washington, D. C, or when personally served on the Contracting Officer or his representative.