The common seal of either Board or of any Council shall not be affixed to any document except pursuant to a resolution of the Board or Council, or of a committee of the Board or Council authorized for that purpose by the Board or Council either generally or with respect to any specified document or class of documents, and the execution of any document so sealed shall be attested by two members of the Board or Council or committee, as the case may be:

“Provided that no person shall be concerned to inquire whether the execution of any document attested by two members of any such committee is within an authority granted under this subsection to the committee by the Board or Council, as the case may be, to execute documents on its behalf.”