If, as I proposed, the water-word *H-p- became postpositions,
preverbs, later prepositions eg. Latin <sub>, <super>, there's the s-
to be explained. Southern proposes to connects it with
the 'adverbial' -s of <abs->, *subs- > <sus-> and with loc. pl.
<-su>. But if this water-noun was used as a postposition in the
Basque style, it would have to be in the locative, eg thus
'<governed noun>-gen.suffix <water word>-loc.suffix'. That the
locative marker /s/ is sometimes prefix, sometimes suffix is
intriguing and indicative of vacillation between prespecifying and
postspecifying in the language.
Or perhaps it's just the /-s/ of the preceding genitive in the
governed noun.
Torsten