This composite word appears to be based on the noun "uso" (dove, pigeon), followed by the diminutive "-nno" to create "usano," meaning a small dove or dovelet. The word is completed by the suffix "-enea" (house) and finally the declension suffix "-ko" (of, from).
Our translation of the inscription is thus: "Here lies dead, on May 1st 1676, Joanes de Saletxe, the son of Maria of Usanone (Dovelet) House." As for a possible Iparraldean or Gipuzkoan linguistic affiliation, this is a question of linguistic geography that archaeologists may be able to resolve by determining the origin of the inscription's stone support.
There are some other items, such as dovelet or doveling, that do not appear in the definition section, but in the lemma, and were incorporated thanks to the revision of other entries and by cross-checking the data provided by the OED with those retrieved from the other lexicographic works, such as A thesaurus of Old English and the MED mainly.
Not so common are peeper, piper, dovelet and doveling.