Fortieth curry in Bow? (4)

I believe the answer is:

ruby

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'fortieth' is the definition.
(a ruby anniversary marks 40 years)

I cannot understand how the rest of the clue works.

(Other definitions for ruby that I've seen before include "Gemstone - shade of red" , "Gemstone comprising mainly aluminium oxide" , "Deep red - transparent gemstone" , "A red jewel" , "Deep red variety of corundum" .)

I've seen this clue in The Sun and The Times.

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