AIR POST: Junkers F-13 over Grate Wall - 航空邮件: 容克斯F-13在长城上空 30c Red

With secret mark

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

30c
Color
?
Original catalog color of the stamp.

Red
Print method
?
Method of process of imprinting designs on paper.

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

12, 12 1/2, 12 1/2x13, 13
Watermark
?
A pattern embedded in a paper during stamp producing process.
— Basic philately terms for amateurs

Character Yu
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