It shall be lawful for Her Majesty, by Order in Council, from time to time, as occasion may require, to direct [F1as regards all or any of Her Majesty’s naval, military or air forces] that all or any commissions for officers prepared or to be prepared under the authority of Her Majesty’s Royal Sign Manual may be afterwards issued without Her Royal Sign Manual, but [F2having thereon the signature of a Secretary of State and, if the order so provides, of a member or members of the Defence Council]; and every such commission issued and signed in pursuance of such Order in Council shall be conclusive evidence that the officer named in any such commission has been appointed or promoted by Her Majesty to the rank or office named therein.