JLPT2 G2 S15 F382
綫 is per 戔# (fine/slight) + 糸 thread → fine *thread*. 線 is a variant form with the same core meaning as 綫 but with the associated nuance of "slender line" → *line* → *clue*; *lead*. In 線, 戔 is replaced by 泉.
[Full Kanji Etymology List]