You're quite right. The name of the car is Catherina, and it was built as an official proposition for a SAAB sports car, designed (as you say) by Sixten Sason and ordered by SAAB to be built at the ASJ factory in
Katrineholm (the name of the car was taken from the place where it was built). Eventually, SAAB decided that more development work was needed, and in the meantime another proposition built by another company, MFI, had been presented. SAAB chose this to be the base for their Sonett II instead of the Catherina.
Are there anybody else that spot the similarities between this and the Fairlady Z?