I'm sure you are aware of the Box, Transit, Gun Machine Carbine .45" Thompson. It has been described before. Some sources state that it was manufactured in America but, as it is identical in construction and fittings with the larger Vickers/Lewis and Bren boxes, I feel this American connection is wrong. The internal compartments housed the gun with butt stock removed and stored alongside plus 3x50 round "L" drums, 4x20 round box magazines, cleaning rod,bore and chamber brush, parts tin containing firing pin protrusion gauge, stripping tool and spare springs/pins etc. and sling. Always described as Aux-Unit chest for safe storeage.
Recently, I was showing some bits to a wartime RAF armourer who was stationed in Burma as part of a Aeroplane Crash Recovery Unit. They didn't do much recovery because they could never find planes that had dropped into dense jungle ! However, their Thompsons were issued in these very same chests and, this happened late 1940 or early 1941. The plot of the box thickens ! Richard Ashley.
I have seen the entry's in the contract record ledger books at Kew for the Thompson chest. They were all made in the UK by a handfull of different manufacturers.
The first order was for 750 chests which coinsides with the first order of guns which was 750 in total.
At the Royal Armouries library in Leeds I have seen drawings for a convertion of the chests to hold Sten gun magazines, I do not know if this was ever put into practice.
I have seen a short film, indeed I have a copy on my computer, of Thompson's being unloaded from these boxes and checked prior to being replaced and loaded into vehicles. The people checking the guns appear to be civilians, some female.
I also have some photos collected over the years of different transit boxes, few of photos added below, and have hoped to find a transit box, but so far no luck.
Have you a link to the film you refer to, featuring the SOE agent in Yugoslavia?
I found this information on an American web site which specialises in Thompson guns:
"An additional supplier, the Educational Supply Association, (ESA) whose products included school furniture which I recall in school in the '60s and '70s.
There is also an order for 5000 placed in India (maker unrecorded).
The initial contract price (which varied somewhat between batches and makers) was nineteen shillings and eightpence ha'penny, subsequently amended to 19/9d.
The totals (in various batches) are:-
Nichols & James 24,601 (including 48 contract overruns which the Ministry took anyway)
Harris Lebus 17,000
for a total of 63,051.
The file page concludes with a note dated 26th June 1941 saying "it is understood that these guns are now regarded as personal equipment and chests are not now required".
Knowing something about the British Army from having done some time in it, this explains why these things were procured in the first place. Unit weapons, which the TSMG was regarded as initially, get a storage/transit chest because they are nobody's personal responsibility - they get issued to whoever-signs for-it-this-time and live in the armoury when the working day comes to an end. Also they have a set of ancillaries (magazines, rod, spares) which need to be kept with it. Lewis guns and Brens and Vickers also had chests, and the Boys a/t rifle, though I've never seen the apparently coffin-like chest for these."