Ilk. *-en suf. “adjectival suffix”
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✶-ina, with the primitive [i] becoming [e] due to Ilkorin a-affection, 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.
Reference ✧ Ety/LAM