Note in Hill
http://www.indogermanistik.lmu.de/VrddhiKonj.pdf
on vr.ddhi derivations
"
wie uridg.*h3nóbh- ~ *h3n.bh-´ 'Nabel'
-> uridg. *h3énbh-e/o- 'Nabelgegend'
so uridg. Inj.-Ind. *h1és- ~ *h1s-´
-> uridg. Konj. *h1és-e/o-
"
in other words, the nominal and verbal stems receive the
same suffix. But that implies that those two stems are
of the same category.
Which leads me to another puzzle: why should both nouns
and verbs stems have athematic and thematic inflections.
And the answer, as far I can tell, is that the bare
verbal stem was syntactically a participle or verbal
noun (which makes the person and number endings originally
deitic particles and the *-i of the primary endings (*-u in
Slavic) locative postpositions (as I've all said before).
Torsten