The design was registered in the second 1/2 of 1933. The National Archives appear to have on-paper detailed information of the registered design. Sadly, not digitised. Designs 784117-788498 | The National Archives
And the date would also indicate some of why the rarity. Assuming it was put in production in 1934, it would only have had a run of 5 years. There is no way this would have been made after 1939 due to material shortages. The advertisement is after the design registration so this is late 1933 at the earliest. Sooo... why the questions? Have you found the unicorn?
Hi Ross, Your memory is correct. I think it was posted in my early picture gallery.Sadly its gone now!!! It will be good to get to Newark and catch up with everybody.