This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
鐽 combines 金 (metal) with 達, which likely contributes the sound. The exact historical development is uncertain.
The left side is metal 金, and the right side is 達 (reach, arrive). Without a known meaning, picture a mysterious metal object that has reached you from an unknown past.
For タツ, imagine a metal dragon (竜, たつ) arriving: 竜 -> タツ, and the metal part hints at its metallic scales.