An adjectival suffix appearing as both -en and -in, and in one place as -on: Brithon. The -en form can be easily explained as a derivative of the primitive suffix ᴹ✶-inā, with the [primitive [i] becoming [e] due to Ilkorin a-affection](/w/short [i], [u] became [e], [o] preceding final [a]), the same origin as the Noldorin adjectival suffix -en. The -in variant is more difficult to explain. At least one example lómen had variations with both -en and -in, so perhaps the two forms represented vacillation on the function of Ilkorin a-affection, or an alternate primitive form ᴹ✶-ină where the final ă was lost before a-affection.
Alterately, -in could be a Doriathrin-specific variant, since the forms where it appears are all Doriathrin, while the forms where -en appear are marked Ilkorin, excepting only lómen which was itself revised from lómin.
Element in: ILK. losgen, ILK. muilin, ILK. ngorthin, ILK. brithon, ILK. Ermabuin, ILK. lómen, ILK. thúren
Apparently an abstract noun suffix appearing in the words Aros and thalos, perhaps of the same origin as the abstract endings ᴹQ. -sse and N. -as, though Helge Fauskanger suggested instead that it is related to the locative suffix ᴹQ. -sse (AL-Ilkorin/thalos).
Element in: ILK. Aros, ILK. thalos