Japanese Naval Signal Flag #1—Code "Hito" 最初の1の日本の海軍艦隊信号-名前「ひと」

There are several types of maritime flags to represent the number one.  Most famously the International Maritime standard uses a flag that looks like the national flag of Japan.  However it is a pennant and the red disc is off center towards the hoist.

NATO has number one flag that looks like the of Spain; because it is a horizontal tribar of red, yellow, red.  Thus its nickname is 'UNO'.

Russia oddly uses the International Maritime flag in a different way for its numbers.  The Alpha flag is the number one flag for Russian allied fleets.  Luckily Alpha is also the first letter of the alphabet, so it makes sense.

But the Japanese Navy has created unique flags.  Their flags have an International Maritime feel, but they are unique to Japan.

The Number 1 flag of Japanese Naval forces has a white background with blue diagonal stripe from the lower hoist to the upper fly (a dexter bend).  In a what the Japanese flag is very effective since the single blue line is a unique flag even among national flags and reflects the single line of one, as it is used in Western and Chinese symbols.  The Indo-Arabic-Western number for one in a single vertical line, while the Eastern symbol for one is single horizontal line.