This poster stamp was encountered some time ago on a visit to The British Postal Museum & Archive (now The Postal Museum).
It was the top-left label of a part-sheet of nine labels. The oval stamp on a second part-sheet read "P.O. Stores Department | May 1931 | Specimen of NEW Form".
Q Was this poster stamp issued by the Post Office? It appears to be part of a series. Who was the printer?