1934 16c Special Delivery Air Mail Stamp Plate Block

Got a plate block of six 16-cent Special Delivery Air Mail stamps from 1934. These are the blue ones with the Great Seal of the US on them. The paper says Scott #771 and it's marked as a plate block with the number 21313 on the side. It's been kept in a HECO protector sleeve. Pretty cool piece of postal history if you're into collecting stamps or just want something vintage. Condition looks decent for its age but check the photos.