An AM REF 10F/94 key.
This is quite a scarce Marconi Air Ministry morse key. An Air Ministry key circa 1910 model AM 10F/94. The overall length including the arm is 5.1/2" by 1.5/8ths wide (140mm by 42mm). Stamped on the base is the Marconi type ID "PS 5785A". Also versions seen labelled "PS 4707E".
Notice the main arm is bent in a slight curve. It would have originally been straight, so this little key has taken some bashing :o)