Thompson muzzle attachment

The square attachment is clear in the screencaps, I was guessing a military blank firing attachment? Other films made in the UK made the time Went the day well which used the same weapons do not feature this attachment. Any ideas? --Maskedweasel (talk) 18:53, 17 July 2014 (EDT)

Can it be a device for attaching a bayonet or maybe a bipod? Greg-Z (talk) 03:15, 18 July 2014 (EDT)