One thousand note Saudi currency is ancient beyond recall (10)

I believe the answer is:

immemorial

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'ancient beyond recall' is the definition.
I can't tell whether this definition defines the answer.

'one thousand note saudi currency' is the wordplay.
'one' becomes 'i' (Roman numeral).
'thousand' becomes 'm' (Roman numeral for a thousand).
'note' becomes 'memo' (a memo is a short note).
'saudi currency' becomes 'rial' (I can't explain this - if you can you should give a lot more credence to this answer).
'i'+'m'+'memo'+'rial'='IMMEMORIAL'

'is' is the link.

Can you help me to learn more?

(Other definitions for immemorial that I've seen before include "Archaic" , "very old" .)

I've seen this clue in the Sydney Morning Herald.

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