The documents and hardware I own don't give me any clue about the 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.
Inside the machine
The PERITEL adaptator daughterboard-card
The RGB signal from this PERITEL adaptator
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]