(7) A material part of a document is every word, letter, figure, seal, or thing expressed on or in a document, or forming part of it or attached to it, and any colouring, shape, or device used in the document which purports to indicate the person by whom or with whose authority or consent, the document or part of it has been made, altered, executed, delivered, attested, verified, certified or issued, or which may affect the purport, operation or validity of the document in any material particular.