Ooooh!
I think qwaszx beat me to the punch, but I'll chime in with more details.
I'd venture that this is the 'prototype' 403 Darl'mat, but since the new 403 platform took so long to develop, Darl'mat built the pretty coupe body on a 203 platform. When the folks at Peugeot found out that the pretty body was being advertised as a "new 403" before the real 403 was available to the public, they were royally angry.
So, it was called a 403 Darl'mat, but in reality it's a 203. Or something like that.