This is a c.1942 Waltham MKVIII this watch was official,y issued as the Wrist Watch MKVIII for Navigators, namely an American 1942 Waltham Model Watch (US 6/0 size ) A 17 Jewel movement within and with the designed ordnance number 6B/ 234
 
These watches were introduced into the RAF in around 1942 when the USA came into the war, it's essentially an A11 Hack Watch which Elgin, Bulova & Waltham were contracted to produce by the US Defence Dept. 
 
However these watches arrived in the UK before the USAAF arrived given British Ordnance status by the War Ministry and were used by crews during bombing missions over France, the Benelux and of course Germany.  These are rather rare, this being only the second one we have had in stock !