Le Coultre continued to sell the Atmos I while it was developing the Atmos II, which the primary improvement was the change from an ammonia and mercury "bellows" to a canister filled with ethyl chloride.By January 15, 1936, Le Coultre announced its "new" Atmos and they were still using the 30" A calibre movement. By November of 1936 the Atmos I production was stopped completely.After which, many end up in the very best homes, because for decades now the world's most celebrated watch-making country has been presenting its distinguished guests with this masterpiece of Swiss artistry.
These prototypes were never sold and never called the Atmos 0 at that time.
His clocks were driven by a "mercury in glass" expansion device which rotated a cylinder which wound the mainspring by ratchet. The clocks are slightly different to the later Atmos models in minor details of escapement.
The Atmos II and the Atmos III have serial numbers ranging from around 4,000 to 59,999 and production went from 1936 until late 1955.
The Atmos V consist of the caliber 526 and the Atmos VI, VII, and VIII consists of the caliber 528 and the 528/1 represents the Atmos VIII. The important point here is that this is the last of the "genuine" Reutter design Atmos made and production stopped late in 1983.
They are very nicely made and typically they have a plate saying "Reutter Brevet" (Brevet = Patent).
In September 1932 Le Coultre entered an agreement to develop movements for CGR and first deliveries were made in mid 1933 and these movements were called the 30" A calibre.
Serial number's for the Atmos II and Atmos III are somewhat intertwined because of Le Coultre purchasing the entire stock of CGR.
There is evidence of overlapping in all Le Coultre models; there are no "absolutes" of serial numbers and caliber numbers.
In late 1983, Le Coultre totally resigned the Atmos and came up with the 540 caliber.