1. CATALOG NUMBER
A modified adaptation of the International Stamp Number [ISN] which was historically-based on the International Standard Book Number [ISBN]. The usual form is [TYPE + YEAR + MONTH + DATE . DIGIT].
The International Stamp Number for Philippine stamps start with a letter P for the country code.

Type [A] denotes
A - for regular postage stamps,
B - for semi-postal stamps, including anti-TB stamps,
C - for airmail stamps, until 1976.
O - for official-use stamps, including G.O. and O.B.
D - for special delivery stamps
S - for souvenir sheet of the regular issue
M - for miniature sheet of the regular issue
K - for booklets, usually definitive issues

Year [XXXX] - the four-digit year of issuance.

Month [A] - the month of issuance:

A - for January, B - for February, C - for March, D - for April, E - for May, F - for June,

G - for July, H - for August, I - for September, J - for October, K - for November, L - for December

Date [XX] - the numerical date of issuance.

Digit [X] - the numeral 1 or 2 is added when different sets are issued on the same day.

2. STAMP IMAGE
When available, a JPEG image of the stamp issued, usually the lowest denomination is shown. When vignettes are different per denomination, a set of images are shown considering space limitations and/or page loading time. When no available image of stamp, the generic logo of this web site is shown instead.