when I look at the photos of the four Atlantic , they are very different . Like one has 3 door hinges , others only 2 . Is there someone who knows all the differences ?

Special the door window, how it looks original, and how to open them.

it looks like if it comes to technical questions the game is very fast over . If it is not the chassis number , the color or one of the owners nobody has any interest .

All Atlantics are vastly different. They are not the same cars! All have a different shape, different chassis, and different details.

_________________
Vive la Marque !!

different chassis ? Can you explain some of the technical differences ?

Aerolithe 1
Aerolithe 2 (did it exist)
and afterwards the T57S chassis.
Body mountings were mostly made in situ, so must have differed as well.

_________________
Vive la Marque !!

all 4 Atlantic have a 57s chassis ? And all have original no window to open ?

is there a chassis number stamped in somewhere ? Or is the number only on the Bugatti nameplate ?

Without going to my references, I know that the forward vent windows on the Ralph Lauren Atlantic (Pope) swing open.

(Matthews book, English, p. 54)

It works the same on the Williamson Atlantic

The Ralf Lauren car , you can swing the front and rear part of the door window . But this is not original . In what year was this change made , and who did it ?
Is there any number stamped in to the chassis to identify them 100% ?

The hinged window on the Lauren/Pope car is totally original, as it was on the Mullen/ Rothschild car. The other two Atlantics, 57453 (missing and presumably the chassis you are trying to find info about) and 57473 had the small side porthole that you were talking about.

in this photo [1936], no hinged window is to see

Well ok, The car left the factory in 1936 with no windows at all, but the following year (1937) the car is photographed at Lord Rothschild's Suffolk estate and the open window on the drivers side is clearly visible. The Pope car, which left the factory later always had the opening widows as far as I could say.

Who made the change , Bugatti ?

Who can explain this ?

Explain what?