Quality of the postcards:

O    unemployed postcard
#    used postcard with stamp
X    used postcard with detached stamp

A    perfect quality (no imperfection)
A-B  very good quality (minimum imperfection)
B     satisfying quality (visible smaller damage)
B-C  bad quality (heavy damage)
C    extremely bad quality (significant damage)

1900    year when postcard was used
(1900)    year (approximately) when postcard was printed