You find in Nordwestblock loans into Germanic an ablaut
a/o: *wa(:)s-/*wo(:)s-, *ha(:)k-/*ho(:)k-
and
e/a: *peg-/*pag- "peg, pole"
(the /e/ of <peg> cannot be due to umlaut or the word would have
been **pedge)
At the same time, from pre-Germanic to proto-Germanic, there was a
development /o/ > /a/, /a/ > /e/.
Timing: Caesar and Tacitus called them Suebi, now they are Schwaben.
The rules (which must be linked) therefore applied after Caesar's
comments.
I think that's the reason for the "ablaut": the various
Nordwestblock words were loaned at different times during the
Germanic expansion into Nordwestblock territory, in the centuries
around the beginning of our era.
Torsten