Centum in Vedic?

Madhav Deshpande mentions in "Genesis of Rgvedic Retroflexion..."
that "some
branches of Yajur Veda show 'kh' where the retroflex 'S' is normally
expected." An example is the famous PuruSa sukta where they
say "Purukhah" instead.

It seems this is common in many areas of Northern India and
particularly with regard to the Zukla Yajurveda.

Asko Parpola in "The Dasas and the Coming of the Aryas" says:
"The Dasas, the first wave of Aryan speakers in Central Asia....
whose arrival at the beginning of the second millenium via
Baluchistan is documented at Sibri and the Quetta tomb. These "early
Dasas" may have developed the "Proto-Yajurvedic" traditions, which
seem to have started blending with those of RgVeda during the spread
of the latter from Swat in the 16th century B.C."

Although this is common all over North India these days, I'm
wondering if we are seeing the remnants of a particular dialect of
Vedic? Was it associated with the Northwest as Parpola hints it might
be?

My main question is whether it is possibly left over from a Centum
form of Vedic since the change from 'k'>'S' is supposed to be more
likely than the opposite?