七

Top horizontal is the indexing radical 一 — files 七 in the radical-1 (one-stroke) section by stroke-count convention. Originally 七 in oracle bones was a horizontal crossed by a short vertical, an abstract numerical tally meaning 'cut and add one' (six-plus-one = seven). The horizontal is the surviving piece of that tally.

Bottom hooked stroke 乚 is a pure abstract numerical mark — combined with the top horizontal it forms the conventional shape of seven. Historically the residue of a short cross-cut after the original tally was rotated and curved into modern script. Neither stroke carries meaning beyond its tally position.