They're wearing the 1922 Scottish Regiments' Service Dress and '08 pattern webbing, plus they don't seem to have respirators handy so I'd put this prior to mid-1939.
Pre-war WD bikes carried civilian (Middlesex series) registration numbers. As well as the short gear indicator (which carried on into 1940), the box has the earlier straight gear lever which points to a 1936 model. Chrome looks to be dull rather than bright and there is no fork adjuster knob.
Although the large deliveries didn't start until 1937, there were about 900 WD16Hs supplied in 1936.