168. According to the grammarians

According to the grammarians, the final s of certain other roots, used as noun-stems, becomes t at the end of the word, and before bh and su: thus, dhvat, dhvadbhis, sradbhyas, sratsu. But genuine examples of such change are not quotable.

a. Sporadic cases of a like convention are found in the Veda: namely, mādbhís and mādbhyás from mā́s: uṣádbhis from uṣás; svátavadbhyas from svátavas; svávadbhis etc. (not quotable) from svávas. But the actuality of the conversion here is open to grave doubt; it rather seems the substitution of a t-stem for an s-stem. The same is true of the change of vāṅs to vat in the declension of perfect participles (458). The stem anaḍvah (404), from anas-vah, is anomalous and isolated.

b. In the compound ducchúnā (dus-çunā) and párucchepa (parus-çepa), the final s of the first member is treated as if a t (203).