Decripting How to Use a 120-Year Old Aviation Watch

This is a story involving a watch, aviation history, horology (the study of time and its measurement) and a forgotten technique for recording time.

The story starts with the acquisition in 1984 of the watch pictured. At the time, all that was known about it was the timepiece was from the WWI and that it was a pilot’s watch. The watch carries only two distinctive marks, both on the movement. One is a serial number (5831) and the other is a Swiss cross.
At first glance, it is a fairly ordinary pocket watch, but two smaller dials (called subdials) and two seconds hands indicate that this watch has a "chronograph" capability. In non-horological terms, the watch both keeps time and acts as a stopwatch, to record periods of elapsed time. The lower subdial indicates the seconds connected to the time and the upper subdial counts the number of full minutes measured by the chronograph. The chronograph is started, stopped and reset by pressing the crown at the top of the watch, which also winds the watch. The watch’s timekeeping is set by depressing a small button just visible on the edge of the case, opposite the 11.

The large seconds hand measures the chronograph’s seconds and this was where the mystery started. Note that this hand runs along a scale that runs from 0 to 300. It did not take long to ask what this scale measured, and it was this question that took nearly 30 years to find the answer.

The obvious path was to start asking the professionals about this. However, every major watch collecting organization and every watchmaker queried between 1985 and 2018 could not come up with an answer. They did comment on what it wasn’t measuring. It was not a tachometric scale, one that allows a chronograph to directly calculate speed or distance based on elapsed time1. Nor was this any of the scientific scales, such as "pulsometer" used by doctors to read the pulse rate of patients, or a "decimeter" which allowed a direct reading of hundredths of a minute.

Even after the internet became popular, repeated internet searches over a period of 15 years failed to turn up anything other than more people asking the same question, but no real answers were available.
This problem remained unresolved until the first clue was uncovered in 2018. A friend noted that 300 divided by 5 is 60, which, obviously, is the number of seconds in a minute. That correlation was a bit confusing until remembering that watches . . .