An idea occurred to me today that I would like to offer for criticism and
comment.
I have long believed that the *o-stem genitive singular, -*�s, should be
regarded as a simple prosodic variation of the nominative singular -*os � a
simple shift of the stress-accent from the root to the post-root syllable.
I propose that the earliest neuter nominative singular in -*om formed a
nominative plural in -*�m, and the neuter genitive plural was differentiated
from it by length: -*�:m.
Subsequently, the neuter nominative plural was re-formed with -*Ha(:),
leaving -*�:m as the neuter genitive plural without a prosodic counterpart in
the nominative.
This neuter genitive plural form was subsequently adopted by the
-*o-stems.