*-T + dh- > *-zdh- ? *-tó- < *-dhó-?

Burrow: The Sanskrit Language, p 90
"
(2) In the case of the combination sonant aspirate followed by -t-
the whole group is voiced and the aspiration attached to the second
consonant; thus from dah- 'to burn' (from dagh- by the second
palatalisation), budh- 'to understand' and labh- 'to receive', the
participles in -tá are dagdhá-, buddhá- and labdhá-. In the older
Avestan language a similar development is observed, though the
aspiration as always in Iranian has been lost: aogədā 'said'
from
*augdha, i.e. Aryan augh- (Av. aog-) + ta, cf. Gk. eúkhomai;
ubdae:na- 'woven' from Aryan vabh-. The later Avestan substitutes
combinations of type (1) above even in the case of the original sonant
aspirates : aoxta 'said', druxta- 'betrayed' (draog- : Skt. druh-)
dapta- 'deceived' (dab- : Skt. dabh-). In the same way in Sanskrit
dhatté 'places' has been substituted for *daddhe (= Av. dazde) which
would be the regular combination of dadh- + te. Elsewhere in
Indo-European innovating forms of this type have completely replaced
the old type of combination : e.g. Gk. ektós : héko:; pústis, cf. Av.
apaitibusti 'not noticing', as opposed to Sanskrit buddhi-.
(3) Dental combinations in Sanskrit normally conform to the above
rules : vétti 'he knows' from vid-, ruddhá- 'obstructed' from rudh-
+ tá, etc. On the other hand Iranian substitutes the sibilant s or z
in these positions : vo:ista: 'thou knowest' : Skt. véttha; hastra-
'session' : Skt. sattrá-; ni-uruzda- 'locked up' : Skt. ruddhá-. The
Greek treatment agrees with Iranian : oístha 'thou knowest', pústis
'information', cf. Av. apaitibusti : Skt. buddhí-. In the Western IE
language -ss- results from the combination : Lat. ob-sessus (sedeo),
O. Ic. sess ' seat'. A tendency to modify the dental combinations is
therefore wide-spread. It is assumed that in Indo-European a sibilant
was inserted in these cases (tst, tsth, dzd, dzdh). Since all
interconsonantal sibilants are elided in Sanskrit an IE voítstha would
produce Skt. véttha, and at the same time it accounts for the Iranian
and Greek forms. In the case of the voiced combination we find two
kinds of treatment in Sanskrit, on the one hand the usual type
ruddhá-, vr.ddhá-, etc., and on the other hand some ancient forms
testifying to the existence of z instead of d as in Iranian : dehí
impv. 'give' beside daddhí, cf. Av. dazdi, and dhehí 'put', both with
e out of earlier az according to the rule below. Either this is a case
of dialectal divergence, or the type dehí (< dazdhí) represents the
regular phonetical treatment which has been replaced in the majority
of cases by new analogical formations.
"

Does the latter paragraph mean (could be interpreted to possibly mean)
that in Skt. eg. the combination -d + dh- > -ddh- went via a PIIr
stage like this: -d + dh- > -zdh- > -ddh-? Because if the *-tó
participle was originally a *-dhó participle which had
allophones/-morphs *-dó, *-tó, later generalized to *-tó, then the
same rule would have applied in that participle: *budh- + -dhó- >
*buzdhó- > buddhá-.


Torsten