Auction #93 Worldwide Air Post stamps and postal history, Zeppelin Flight items, philatelic rarities of the World

1913, Tercentenary of the Romanov Dynasty, Romanov Castle, stage proof of 3r in black blue, engraved printing on thin creamy paper, mounted on sunken white card, proof size 45x33mm (top right corner clipped), card size 71x50mm, slight traces of mounts, VF and extremely rare, according to Robson Lowe the only card produced, which is similar to proof on India paper made by ABN for US and Canada, Est. $1,800-$2,500