It’s a manual computer for use with the Mark 14 wartime bombsight as fitted to British heavy bombers. Without digging the books out i think it was located on the Lancaster in a small slot underneath the Bomb aimers kneeling cushion so that it could be used in an emergency if the main bomb computer packed up.

There are quite a few of these about and maybe ‘Air Ministry’ might be able to add some more information but my gut feeling is that these maybe early post war issue as the AM stamp has been replaced by the Crowsfoot.