(EN) Jean-Paul G. Potet, Baybayin, the Syllabic Alphabet of the Tagalogs, Lulu.com, 2019, p. 115, ISBN 9780244142414.
«but an examination reveals that they cannot be earlier than the 17th century because in the excerpt shown here, the letter nga (frames 1 and 3) has the /a/-deleter cross that Father LOPEZ introduced in 1621, and this cross is quite different from the diacritic placed under the character ya to represent the vowel /u/: /yu/ (frame 2).»
