An 1886 Chinese Large Dragon cover sent from Tientsin to Cassel, Germany is valued at $490,211-645,015 ahead of a sale at InterAsia Auctions.
The lot will headline the Asian Stamps & Postal History auction in Hong Kong on July 18-20.
The issue, the first Chinese postage stamp, entered circulation in 1878. It was established by Britain's Imperial Maritime Customs, which shipped post to treaty ports throughout Asia and around the world.
The auction house describes the lot as "A gem for the connoisseur and a highly important Large Dragons on cover registered usage of the utmost rarity and highest exhibition calibre."
An 1897 red revenue small figures surcharge 2c on 3c block of four is likely to prove another highlight, with a valuation of $154,797- 193,497.
The lot is one of only nine recorded examples of this block in existence and is likely to prove very popular with bidders.
InterAsia describes it as "a truly extraordinary and outstanding multiple of this rare variety".
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