> > > And we can add to this oddness the greek hebd- besides hept-.
> Of course. So, there is a triplet of variants: *septm / *sepm / *sebdm.
The Greek voiced form may simply be by analogy with the voiced forms in
"eight": ogdoos etc. The origin of the voicing in 8 remains obscure, but
laryngeals have been dragged into it. It is one of the bits of evidence
sometimes used to prove that h3 was voiced. The presence of a laryngeal at
the end of 8 seems clear (Sanskrit shows -au and -a, and cases with
both -a- and -a:-), and the ending -o: points to -h3.
Peter