The following article mentions the six verses of the Ṛgveda that have no Padapāṭha (on p. 44):

Kashikar, C. G. (1951): "The problem of the gaḷantas in the Ṛgveda-Padapāṭha." Proceedings of the All-India Oriental Conference 13 (1946), 39-46.

Commenting on this, I wrote in 1981 (p. 87): "This is most easily explained by the assumption that these verses were not considered part of the Ṛgveda by Śākalya. It further shows that the final redactors did not hesitate to deviate from the composer of the Padapāṭha in deciding what did, and what did not, belong to the Ṛgveda. (It is interesting to note that at least one hymn of the Ṛgveda (10.95) is known to have had fewer verses than at present at as late a date as that of the Śatapatha Brāhmaṇa. See Oldenberg, 1912: 303.)"

"The orthoepic diaskeuasis of the Ṛgveda and the date of Pāṇini." Indo-Iranian Journal 23 (1981), pp. 83-95.

http://my.unil.ch/serval/document/BIB_F9751BB34E62.pdf

with a reference to:

Oldenberg, Hermann (1912): Ṛgveda. Textkritische und exegetische Noten. Siebentes bis Zehntes Buch. Berlin: Weidmannsche Buchhandlung.

Johannes Bronkhorst