Questions? Looking for parts? Parts for sale? or just for a chat,
The WD Motorcycle forum
'GMD' was a Middlesex series issued June - July 1937 but not specifically one of those reserved for the War Office. If you're able to share the photo, I'd expect that the combined forum resources could give you chapter and verse on the motorcycle.
I will try and upload the photo
email (option): Shockingwhite@hotmail.com