seven and such

I had this idea on cybalist some time back that IE and AfroAsiatic
six and seven shared a prefix *s(a)- since IE has s-less forms
of 'six' (eg Prussian 'uschsts'), and since Malayo-Polynesian
has 'pitu' "seven" (Arabic 'sabattuN').

Acc. to Bopp 'sa' in Malay is "one", and is used eg. as

kapâla sa batu "a head", lit. "head one stone"

which makes one suspect a classification system ("one piece head").
So 'sa-batu' = "one", but one what? Something that was later left
out? IE numerals over 4 behave like nouns (except they are not
inflected for case; Russian consrtucts them with gen. pl. as if it
were eg. "a seven-ness of [whatever]").

In Javanese, another classification word is used with 'sa', namely
in 'saviji' and 'siji', which contains another classification noun,
which means "seed grain" and which is similar to Sanskrit 'vi:ja'
(acc. to v. Humboldt).

Now why do these two classification noun contructions look like IE
and AfrAs 'seven' and 'six'?

Torsten