Locative of tanU

[Dmitri]

Dear Indologists,

Could someone advise me on the following problem:

Word tanU (MW 435.2) has loc.sing. as tanvI couple of times in RV.
In taittirIya-saMhitA there appears to be one occasion of
tanuvi form of the same case  --- in TS 4.3.13.1d
(according to Vedic Concordance)

The question is: could the form tanuvi be considered as valid, or does it
appear in TS only due to some special circumstances?
Could it be assumed that tanuvi occurs in other MSS?

Best regards, Dmitri.