We talked about some of these mounts on my restoration thread linked below. Scroll down to the Carrier Candy section on page five and message 150 has measurements of the Bren mount which fits into the cast sockets used with the pogo sticks.
http://www.mapleleafup.net/forums/sh...?t=9136&page=5
I've seen the curved goose neck type mount in Jordan's photo somewhere else. Will see if I can dig it pics. Seems like it was on very early modified jeeps that the British airborne first had when they were still in the "show off" public relations mode prior to anything being done operational. Circa early 1942.