Having found it at last, I've been through nearly 200 pages of pics in Schiffer's Motorcycles of the Wehrmact by Horst Hinrichsen....and it's not in there!I am trying to google an image or two of them in Wehrmacht service, they were rare beasties
Markus, that's a very good candidate...but look again at your pic of the Ardie - do you notice a major difference with the motorcycle in the original pic?
In the original wartime pic - there's an oddity; in behind the front wheel, in front of the engines - there's a magneto/dynamo, probabbly drive by a chain on the right side. You can see the "weather shield" over it...
In your pic, the 1929 Ardie twins had had this moved by their engine supplier - they weren't making their own four-stroke motors inhouse from 1925 - for a ten year period or so, they bought-in JAP engines (that's J.A. Prestwich, not the nationality!) from the UK; JAP was a HUGE pre-war supplier of proprietary V-twins and single engines for motorcycles and cyclecars. Up until 1928, the mag/dyno was in the vintage position, in front of the engine...here's a 1928 JAP-engined Ardie 500 single...
Now, here's a 1930 500 single - see the chain case relocating the mag/dyno to behind the cylinder for that year?
Ardie catalogued its last v-twins in 1929 - like a lot of motorcycle companies around the world, the Crash of '29 saw a sudden winnowing of luxury models!
By the way - has anyone noticed the small oval name badge that's been painted-over on the petrol tank?