In my professional opinion, the following item is:

#C6 Biplane Airmail. Top Plate Block of 6.  

I grade this item: Very Fine 3, which means:

Very Well-Centered. Original Gum. Wide selvedge.  Good color and impression.  Two hinge remnants.  

Scott Catalog Value: $1700

Thank you,

Karl Anderson

Anderson Stamp & Coin

POB 51258, Provo, Utah 84605

APS #130744.  I am a full-time stamp and coin dealer; and a regular advertiser in Linn's Stamp News and Coin World Publications (since 1975).