French expired passport records for CNF by descent application

Hello,

Like many others, I'm currently a bit stuck trying to obtain my CNF - Ibapplied in 2019 and have just had the pleasure ofbeing invited tobthe consulate to be handed a letternof refusal on account of a missing document! I have a French mother, born in France but moved to the UK in 1958. She was on her French mother's Livret de Famille at the time of moving, and has a French birth certificate and a current French passport but because she's lived out of France I need to show some other proof of French nationality between 1958 and 2008 (the 50 year rule I think).

She previously had a French passport issued it some point between 1958 and 1978 (her memory of that time is not the best), this expired and wasn't renewed qt the time and she has since had a new French passport issued in 2019. The problem is we can't find her old expired passport anywhere! It's possible it got completely lost in a recent move, sigh I have contacted the French consulate and they say they don't keep archived records and have directed me to the French archives, although this seems to be mainly for historic documents - I haven't yet contacted them.

It's a longshot, but does anyone have any experience of, or advice for, getting hold of records of expired French passports? Perhaps it just isn't possible, in which case I think my application is dead in the water sadly, but I don't want to give up just yet...

Appreciate any help or suggestions.