They are quite small and have the major adjustments of gap spacing and key pressure easily altered externally, which suggested that they were not used by regular operators such as ship's radio officers. One suggestion, from the Marconi connection was that perhaps they were for marine use in lifeboats.
Following enquiries via a number of special interest groups, someone found them illustrated in the Marconi catalogue for 1953 as being an accessory to the AD107 aircraft HF transmitter. The key illustrated must have been a prototype, since it differs in minor detail to those in the field.
This explains the light weight design and external adjustments as different operators would have used them on each flight. This key is unusual in having no rotating bearing with a beryllium-copper spring providing the flexibility of the main shaft. My keys have inspection stamps suggesting that they were used in RAAF rather than commercial aircraft.
The difficulty in finding information implies that they are probably not very common.
Count since 1/10/09.