[Bevdeforges]

Not sure how far back the records go, but you should at least ask the office in Nantes for a "French birth certificate" for your father. https://www.service-public.fr/particuliers/vosdroits/R1405

If your father was registered as being French at birth, he should have a birth certificate in Nantes - which would be the "A ticket" as far as proving that he was a French citizen.
Cheers,
Bev

 

[Bevdeforges]

Everyone born outside of France who has or who takes French nationality is supposed to have a birth certificate at Nantes. Since the demand is a freebie, just fill in the form and request his (use that number if you think it might help). It may go quicker than you think.
Cheers,
Bev

 

[Bevdeforges]

Does it not help that I had a passeport before as well as a french identity card do they keep records of these?

They asked me to send a copy of a police report as to the lost items, which I have done.

EH beat me to saying it, but most countries have a "renewal" procedure if you are renewing a passport that has expired "recently" and can provide the expiring or expired passport. That is limited, however, to passports that have expired within the last 5 or so years. To request a passport after that time, whether you have the old passport or not, is in essence a brand new passport request and is treated more or less like you never had a passport before. You have to prove all your citizenship and identity stuff from scratch.

I'm sure this is done to prevent people from finding an old passport and then "renewing" it in order to change their identity or for other nefarious purposes.

They may check on your prior passport and/or identity card. But as far as they are concerned, they are issuing them anew and require all the documents you had to provide originally.
Cheers,
Bev