On pg. 439 in discussing the origin of the p.p.p. in -itus < *-etos of 2nd conjugation verbs I wrote:
At an early date, probably in western dialectal Proto-Indo-European, (fn. 55) the following analogy took place:
*kap-ye- : *kap-to- :: *moneye- : X, X = *mone-to-.
In other words, the suffix *-eye- was reanalyzed as stem in e- plus suffix *-ye-. In the formation of the verbal adjective in *-to- the suffix *-ye- was truncated.
In fn. 55 thereto I wrote: The same analysis and analogy is evidently reflected in the Germanic past participles of the first weak class, e.g. Goth. nasiÞs* ‘saved’ < *nosetos.
This is not quite sufficient and not entirely correct either. First, it was not just the p.p.p. that the stem in e- is found but also in the perfect active in -uī from *e-w-ai, e.g. monuī < *mone-wai. Second, the Gothic evidence is inconclusive since i can of course reflect either *e or *i and in general the union vowel of p.p.p. copies the stem vowel of the present. As a matter of fact the isolated ON mettr 'full, satiated' from the verb *matija- 'to satiate' with i-umlaut points to a proto-form *matiđa- with an *i. So it seems that Germanic did not partake of this innovation. Third, Celtic did partake in this innovation. This is clear from Old Irish W 2 a verbs (old iterative-causatives) which contrast raising in the present stem, e.g. do luigi < *-log-ī- < *-logeye-ti vs. no raising in the s-preterite, e.g. do loig 'forgave' < *-loge-st, and in the pret. pass. -logad < *-loge-to-, an old verbal adjective. Identical facts are seen in Middle Welsh where ī-presents with o-grade roots have pret. stems in -es. This seems to be a a significant but under-appreciated common innovation of Italo-Celtic. See Schulze-Thulin 2001:86-9 with earlier literature.
Schulze-Thulin, Britta, 2001. Studien zu den urindogermanischen o-stufigen Kausativa, Iterativa und Nasalpräsentien im Kymrischen. Innsbruck: Innsbrucker Beiträge zur Sprachwissenschaft.