A board of education, with the permission of its chair, is empowered to authorize, with the consent of the comptroller of the treasury, the use of mechanical signing equipment approved by the comptroller of the treasury, to affix the signature of the chair of the board of education and of the director of schools to the original of a public school warrant; provided, that a clear duplicate of the warrant is kept on file in the office of the director of schools, together with the proper supporting papers to justify the issuance of the warrant.