When did diacritic signs 濁点 and 半濁点 start to be used in the Japanese language?
I heard notation method of them were enacted in the period of Showa after WW2.
This is a post card that was made before Showa era. はがき is written as はかき on it.
However, actually it seems that they are used since long ago.
It seems that 濁点 was used since Hiragana was invented in the 10th century and that 半濁点 was invented by Portuguese missionaries in the 16th century.
Originally they were used as supplementary symbols for pronunciation.
The 4th line from the right of the second page can be read as 「悪を退け善に勤しむべき事」. 「べ」 has 濁点.