The documents and hardware I own don't give me any clue about the official release date of this PERITEL Atari 400 computer in France, but I'll assume it was released alongside its sibling, the PERITEL Atari 800.
To read about the PERITEL adaptor and the resulting output, I invite you to visit the PERITEL Atari 800 page. I will not duplicate information on this page describing the 400 model: it is also a mod of a PAL Atari computer, with a different but equivalent daughterboard-card.
Note: the antenna cord was removed on this one but it is usually there and continues to work. See the family pictures (below) with the other ones, the antenna cable is still there.
I am the happy owner of 3 of these machines too...
Inside the machine
The PERITEL adaptor daughterboard-card
The RGB signal from this PERITEL adaptor
Using 'PEEK' commands in BASIC to find out about the OS, Basic and GTIA versions.
For your information, the results are:
PEEK(53268) = 1 [PAL/SECAM]
PEEK(65528) = 214 [400/800 OS Rev. A/PAL, 1979-06 (common)]
PEEK(65527) = 255 [Not a XL/XE]
PEEK(43234) = 162 [Atari BASIC Rev. A, in external cartridge]