According to Cross & Freedman, _Early Hebrew Orthography: A Study of the
Epigraphic Evidence_, pp. 46-48:
The Gezer Calendar (c. 925) does not use them. (The waw in YRXW is not a
mater lectionis, but is to be understood phonetically as "ew" (contraction
of
"ehu", and the word translated "his two months".)
The Shemaiah Seal (9th century) has the word LSM(YHW "belonging to
Shemaiah": "The final vowel u is represented by the vowel letter waw. If
the dating
of this seal is correct, this is the first instance of a mater lectionis in
Hebrew orthography." They also note: "The fact that a full system of matres
lectionis was employed in the Me$a` Inscription from the middle of the 9th
century, suggests strongly that they were being used in Israel at the same
time."
[b-hebrew] dating of matres lectionis,
Bearpecs, 07/25/2005