Laycock, Donald. Three Upper Sepik Phonologies. 1965. wals:3490
Glottolog lists this source as "Iwam unclear which one probably iwam1256".
/ɾ̃/ is described in the source as a velar nasal /ŋ/ with an "alveolar flap [n̆]" allophone that appears initially and intervocalically but not finally, but taking the word-final allophone as the underlying form is maybe a stretch.
Vowels are usually nasalized when adjacent to nasals and "frequently initially and finally".
Initial clusters are Cw for all non-coronal C but /w j/, as well as /nw pr kr hr/ and possibly /hn/. Final clusters are /w j/ + /p t k m n s r/, as in _əwt_ 'cane'.