A late-war Dalton Dead Reckoning Computer Mk.IIID*.6B/180.

Late war RAF Dalton Dead Reckoning Computer Mk.IIID*.6B/180.

The model letter suffix * indicates that the circular scale is rotated by grasping it. In the original Mk III the scale was rotated by a knob at the side of the instrument, but this probably proved too complicated when wearing flying gloves at night.
Models A,B,C and D were designed to be strapped onto the leg. A&B was for slow speeds, C&D for higher speeds. In each case the first (A & C) were based on an isothermal model of atmosphere and the second (B & D) on an ICAN atmosphere.
This example has a Broad Arrow marking, and retains its original "Bound with any paper that is available" notebook.
(The outer covers appear to have come from a 1944 "Everybody's books, London," pulp fiction anthology "Tales of Terror and the Unknown"... the specific portion being from the short story: "Strictly to Plan" by Preston Yorke; which was the pseudonym of Harold Ernest Kelly; who also used the pseudonym: Hank Janson; the most popular and successful of British pulp fiction authors of the 1940s and 1950s.)

The "A M & Crown" was replaced by the Broad Arrow in 1944, when the Ministry of Defence was created to oversee the armed forces of Great Britain. The Dalton has also had its leg straps removed so that it could be used as a "Stand Alone" instrument on the Navigator's chart table.
The leg straps were dispensed with as standard with the introduction of the Model F.

Only models A,B,C,and D. appear in the 1944 RAF manual of navigation; which. together with the other clues, suggests the date of this example is indeed 1944.