AIR POST: Junkers F-13 over Grate Wall, Surcharged - 航空邮件: 容克斯F-13在长城上空, 套印的附加费 $53 on 15c Gray green

With secret mark

Country China
Denomination
?
Face value of a stamp, usually printed on the stamps.
— Glossary

$53 on 15c
Color
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Original catalog color of the stamp.

Gray green
Print method
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Method of process of imprinting designs on paper.

Engraved
Perforation
?
Holes punched between stamps to make separating easier.
— Basic philately terms for amateurs

13, 13x12, 13x12 1/2, 12 1/2
Watermark
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A pattern embedded in a paper during stamp producing process.
— Basic philately terms for amateurs

Unwatermarked
Issue Date
?
Date inscripted on a stamp or cancellation for used stamps not necessary correspond to real date of stamp issue.
#
?
Catalog number of stamp.
— Basic philately terms for amateurs