Sanskrit and e, a, o

According to the good old "Sanskrit consonants and Greek vowels"
rule, PIE had a vowel (how come this "Ablaut vowel" doesn't have a
name?) which was realised as e/o/nothing. For PIE-internal reasons
(how to plausibly relate e and o phonetically?) Cuny slapped on a pre-
proto stage where the Ablaut vowel was ä/a/nothing. More plausible.

Some IE languages underwent, in the traditional framework, o -> a.
Let's call them group I. The rest had no change for PIE o. Let's call
them group II. Now suppose we reversed the change/no change. Then
we'd have

PIE ä/a/nothing -> e/a/nothing -> (Group II only) e/o/nothing

And BTW writing e/o/nothing is rather cumbersome, especially in PIE
roots. Would eg. "A" do for a name for "the Ablaut vowel"?

Torsten