I got my sister a Atmos VIII as a present a few years back, and it took us a few attempts to get it running at the proper speed via the adjustment lever (the analog world is messy). Now it gloriously stands there, never needing to be touched to keep time.
Professionals have instruments that measure the precise period of the clock ticks that are heard, so the adjustment lever can be set accurately immediately.
With a computer (or with a smartphone), one could easily improvise an equivalent instrument, by recording the ticks with the microphone and displaying the tick period during adjustment.
I’ve seen so many of these clocks at antique stores, estate sales and such with with IBM plaques on them with someone’s name. Apparently IBM used to gift these to employees for 25 years of service or something to that effect. Given the cost and the potential longevity of the clocks it’s quite a statement for a loyalty gift.
Incidentally, if you want an interesting pre-Atmos model, look up the Beverly Clock. It's been running since 1864 and was built in somewhere that would have been jungle-like bush a few years earlier given that New Zealand was barely settled at the time, it's older than the institution that houses it.
It wasn't just IBM, it was a standard long-service/retirement gift for US corporate execs, to the point where I think a significant percentage of Jaeger-Lecoultre's income at the time would have been from shipping Atmos clocks to the US. The leftovers are all over eBay as the descendants sell them off, you can tell them apart by either the engraved plaque on them or the two screw holes where the plaque used to be. However they're also quite a risky buy, they'll never have been maintained and probably dumped on a garage shelf for several decades so can be in pretty poor condition. As a result your USD200 eBay find could end up costing USD1500-2000 to be shipped back to Jaeger-Lecoultre and rebuilt. If anyone's thinking of buying one, ping me in a reply, but a general suggestion is to get the service record and history of how it was stored before buying. You'll pay several times as much as the found-in-granddads-attic ones but you also won't have to pay to get them shipped to Switzerland for a rebuild.
I wonder why Chloroethane / ethyl chloride was chosen. I don't think I've ever seen this for anything, other than a synthesis precursor to useful stuff.
My guess: it is a relatively safe compound with the right properties to act as a working fluid for this movement. The text linked below mentions the working fluid and the property that the vapor pressure is highly variable over the range of normal room temperatures.