Hi James,
As you can see from the info posted above, it is the later Mounting, Straddle, Bren, AA. I had the earlier "over the shoulder" mount in mind when I first replied.
Googling this term I came across the attached picture from a February 1944 War Office publication "Identification list for the mountings Straddle AA Bren and .303 Browning". Clicking on the link however, led me to an eBay listing for the
Bren 1948 Light Machine Gun Training Manual (Canadian) (no connection to seller). I could not see the attached picture there and I doubt this contraption was kept in service after WW2. However, it may have just survived in service long enough to by put into use near Calais in the summer of 1944.
Also see the thread
Bren LMG mount and scroll down to see the surviving example in the Netherlands Army historical collection.
