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成 supplies the sound - chéng directly for the verb reading 'to ladle into a vessel', and chéng shifted to shèng for the abundance reading.
成 itself pictures a halberd-and-stake, an image of consolidation reaching fullness - that flavour of 'brought to completion' faintly reinforces both senses: a vessel filled to the brim, or a state reaching its peak. Same phonetic in
城 (city) and
诚 (sincere).