It's a nav aid/device known as the Dalton computor ( computer ) and invented and developed by an American, Philip Dalton in the 30'. Adopted by the RAF and used during WWII. There was a couple of versions made. Depending on the model, the time of making/ introducing/using may also be different. For instance the model of Ref no. 6b/145 Mk.IIID seems to be of 1939/40 while the Ref no. 6b/180 Mk IIID* of 1940/41. The Ref 6b/214 Mk.IIIF* of 1941/42. The Ref 6b/250 Mk.IIIH* and the G seem to be the later computors. The year of production is written with the serial ... eg ... 4144/39
The models A,B,C and D were made to strap on the leg. Mk.IIIA and B were for slow speeds, the C and D for higher speeds. In each case the first A and C were based on an isothermal model of atmosphere and the second B and D on an ICAN atmosphere. Model F was made without the straps but with some scales on the reverse and made to stand alone on the chart table.
To sum up ... there is no clue when the RAF adopted the kind of the nav help. However the Dalton Computors and their derivatives were being used quite widely in 30' , the WW2 time and later. Therefore the early models, I mean the A,B,C and D ( mostly the C and D, IMHO ), could be used by the RAF from 1939 and later. Just my opinion.
thanks, my friend was a Royal Flying Doctor pilot in Australia during the 60's & 70's - says he used one then. Considering loaning mine to a travelling museum display of 1920's \ 30's air history if it's appropriate for that time-frame.