Miguel wrote:
<Celtiberian -o (not an instrumental, which is -u, nor
<an ablative, which is -uz) in my view represents *-os(y)o >
<*-o(y)o > *-o:, and is thus fully comparable to Greek -ou (=
</-o:/) < *-o(y)o < *-os(y)o.
Another explanation is analogy:
*sosya:s touta:s "of this tribe" : *sosyo wiri: "of this man" > *sosyO wirO
i. e. *-sya:s : *-syo = *-a:s : X (X = *-o)
Mate