Machine-readable code for retail purposes (3,4)

I think it could be either:

Which length description are you looking for?:

3,4

bar code

(Other definitions for bar code that I've seen before include "Stock item marker" , "Rulebook for lawyers maybe" , "Computer-read supermarket lines" , "Brocade (anag.)" , "that needs scanning" .)

I've seen this clue in The Guardian.

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