A 20mm gun mounted on a Universal Mounting on the back of a CMP 15 Cwt, War Department number Z 5194984. Photo - Andy Morgan
A 20mm gun, possibly the same one as shown above, shown deployed on the ground. Photo - Andy Morgan
CMP page on 20mm 15 Cwt from AEDB Design Record
http://bcoy1cpb.pacdat.net/20_mm_aa_guns.htm
For a long time people told Dave Ballard, the owner of a restored 15 Cwt AA truck (the ONLY one restored apparently) that these trucks never made it over to the Continent for action during WWII. I quashed that with a photo showing a bunch of the CMP 20 mm AA trucks on the beach in Normandy in July 1944. They appear to be British 7th Armoured Division from the markings. Unfortunately the example on my web page won't load full sized image at this time. Pictures of Dave's restored truck may be seen at:
http://www.mapleleafup.org/beltring01.html
Here is the British Airborne 20mm Polsten TOWED mounting. These were towed by jeeps. I found a restored one for my jeep - but could not afford it.
Mounting, Airborne, 20-MM, Gun No. 2 Mk. 2 Land Service (shown without the gun - illustration from the manual)