That in addition to the mode in which such instruments may now be executed in this state, hereafter all deeds and other instruments in writing executed by any person or by any private corporation, not having a corporate seal, and now required to be under seal, shall be deemed in all respects to be sealed instruments, and shall be received in evidence as such, provided the word "seal" or the letters "L.S." are added in the place where the seal should be affixed.