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.
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. This transmitter and its companion receiver, the AD108, were introduced in 1949, effectively replacing the T1154/R1155 and were fitted to both service and civilian aircraft. 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 radio operators would have used them. 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 the Australian airforce, possibly in Canberra bombers which used them in the RAF.
The difficulty in finding information implies that they are probably not very common.
Count since 1/10/09.