Hi Simon,
I found this on the Bonhams website, which doesn't answer your question, but may be of some help:
The chronometers were fitted with Mercer's patented control device (Pat. No. 26/261655) which sends electrical impulses once every second and also every quarter second. These machines were purchased for use as the timekeepers for transponders in aircraft. Simultaneously sending out a one-second and quarter-second signal allowed the aircraft to be tracked by using radio ranging. The eight-day duration fusee movement allowed for uninterrupted and precise transmission.
Bonhams 1793 : An early 20th century eight-day mantel chronometer Thomas Mercer, St. Albans, numbered 705
Details of the patent here:
Espacenet - Bibliographic data
The description "use as the timekeepers for transponders in aircraft" seems a bit ambiguous. My guess is the chronometer would have been used to send time-signals to a ground transmitter.