end of thread, thread, clue
緖 centers on the beginning or guiding part of a thread: the end of a thread that leads to the rest, and by extension a clue that leads to a solution.
緖 combines 糸 (thread) with 者, which likely contributes the sound and the idea of a starting point or a person who begins something. The exact historical development is uncertain.
The left side 糸 is thread, and the right side 者 looks like a person. Imagine a person holding the end of a thread, which is the clue to unraveling the whole spool.
For ショ, picture a short thread: short -> ショ, and the short end is the clue that starts the unraveling.