75mm Airborne M1A1 Pack Howitzer (1942) designed to be dropped into battle with airborne troops.
This one was used by 1st Airlanding Light Artillery Regiment, Royal Artillery, at Arnhem bridge September 1944 where it was damaged (see broken tow ring).
It is now in the Royal Artillery Museum, Woolwich, London.
UPDATE: The Royal Artillery Museum (aka ‘Firepower’) in Woolwich closed on 8th July 2016 and had vacate the site by the end of 2016. The development of a new purpose-built heritage centre close to the Royal Regiment’s home at Larkhill on Salisbury Plain is in the early stages of planning. For the duration of the transition period they are storing the majority of the collection at a site owned by the Science Museum Group in Wiltshire. It is not open to the public while in storage. Follow developments on their Facebook page.