Arctic 1931 Wilkins Ellsworth Arctic & North Pole submarine cover. Scarce registered

GB to USA registered cover. Carried by submarine under the North Pole in 1931. Sir Hubert Wilkins was an Australian pilot who pioneered many Arctic and Antarctic flights and routes. He was knighted for his efforts. This cover is in Sir Hubert's handwriting. Signed by Wilkins.
Sent with Standard Delivery - Registered