*st@dhlo

I've been reading Szemerenyi's Introduction to Indoeuropean
Linguistics and I've come upon what appears to be a odd
reconstruction. It concerns the word *st@... (stall), and he only
gives Lat. stabulum as a reflex. Two things struck me as odd, the
reconstruction of schwa and the reconstruction of dh. If Latin were
the only source language I would expect these sounds to be /a/
and /b/ or /bh/.

Now my sources are somewhat limited and my knowledge of the
historical changes also limited, but I couldn't find any obvious
reflexes of *st@... in any language. OTOH, I could find several
words that appear to come from something like *stab(h)lo. To make
matters worse I consuled two dictionaries and both agree that Lat.
stabulum comes from stare (which of course comes from *sta:).
Furthermore I couldn't find st@... in Pokorny's list.

So I'm hoping that you experts out there could clear this up for me.
Is there any basis for Szemerenyi's reconstruction? If he is in
error, what is the correct reconstruction of the apparent cognates:

English stable
Irish stable
Greek stablos
Latin stabulum (?)

Could *st@... be cognate with stadium?