Miguel:
> What we find is an ins.pl. in *-o:ys, which _could_ mean
> that the "plural" *-s _did_ have a lengthening effect,
Well then, we could say that *m has a lengthening effect
in the thematic gen.plural too. Of course, that line of
reasoning would get absurd and everything under the sun
then might cause lengthening. The gen.pl is the result
of disambiguation from the otherwise identical accusative,
I'd say, but some would contrastively say that it is the
result of contraction of *-o- + an ending *-om that never
lacked its vowel. Whatever the case, perhaps the above can
be explained similarly.
= gLeN