It depends on year as well as the country of manufacture.
For cars made in France I think the change from recessed chassis rails to straight ones was in the 1970s. I don't know which year, but it would have been after 1973. I guess the change was around 1975.
For cars made in Spain the recessed chassis rails were still in production in 1984. French cars had the straight rails before 1983.
I don't know why they ever had the recessed chassis rails. They are interchangeable. An early body can be fitted on a later chassis if you remove the metal under the luggage compartment floor designed to meet up with the earlier rear chassis rails.
