Crossword Genius

Not even once speech was curtailed on a numerical system (5)

Ross

I believe the answer is:

octal

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'system' is the definition.
Both the answer and definition are singular nouns.
Perhaps there's an association between them I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for octal that I've seen before include "Pertaining to a number system with 8 as its base" , "Working in base 8" , "Of a number system with base eight" , "based on eights" , "(Of maths) in base eight" .)

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