I found this ruck sack yesterday at the carboot sale it's 1944 dated with makers mark of M&S. The writing on the front looks original. I will post better photos soon. It says pack auxiliaries 20mm.mountings mk 1
The purpose is sufficiently obscure to be genuine. The British Army dabbled with light AA guns just at the point they were no longer really needed. Good find!
The 'Mountings' is the key word as there were several different guns that, I believe, shared common mounts. Ah-ha: 20mm AA Guns – www.captainstevens.com