The car appears to have had some mild customisations. Though, generally speaking CD Cosmo (fastback coupe that debuted in 1975) was also badged as Rx-5 and 121 in different markets, essentially the same car. That said, one significant style change, was the introduction of the notchback 'Landau' version (coined Cosmo-L or 121-L) in mid 1977 - much fewer in production numbers - and this is what we have here.
Until it was superseded by HB Cosmo / 929, both fastback and Landau shapes of the CD ran until early-mid 1981, with the series II models (from mid '79 onwards) getting a minor upgrade to cosmetics (and mechanics).
An interesting ride you've got there; a bit of a mild custom ...
Being a 1978 (or even early '79), and based on the picture of the interior, I'm confident it's a series I Landau, but sporting series II bumper bars.
The full one-tone roof is a unique look (as it would have had a 1/3 vinyl roof originally; hence the namesake 'Landau').
Its side indicators and tail-lights are not 1975-1981 CD Mazda Cosmo (121/Rx5/121L), and I'd suggest the tail-lights (and especially the number plate surrounds) are not even Mazda (they look familiar, but I can't place them ... later model Toyota perhaps?!?).
FWIW: While the series II (mid '79-'81) Landau got an update to the nose (grill, bars and lights) and the dash/interior, as did its Fastback Coupe sibling, the Landau did not get any treatment update to the tail-lights, thus left sporting the tail-lights of the series I. Perhaps that was one of the motivations behind the previous owner's customization of the tail-lights.
All the best with the project though; the effort on such classics is rewarding (mentally, and nowadays financially too).
