My copies of the British 1940 Small Arms Training Pamphlet No. 21 is also not to hand, however there are some pictures in this post:
It does contain pictures of TSMGs with both horizontal and vertical foregrips... and the Thompson in the photos is a Colt era example, which can be identified by the markings on the receiver.
The date on the cover of British 1940 Small Arms Training Pamphlet No. 21 is July 3, 1940 and yet it is a Colt TSMG with horizontal grip used for illustration purposes. In the same month, the first sighting of the Savage MODEL 1928 with Cutts, L drum, and horizontal foregrips is in the British Pathe film "This Is A Tommy Gun" released July 11, 1940. The William Clowes & Sons Ltd. did not have a Savage TSMG available before they went to press?
Perhaps it was just 'pot luck' and the first one that came to hand, when they were looking for a model to photograph, for inclusion in the pamphlet. I would think that they were not concerned with, what at the time were 'minor' details such as manufacturers markings.
We can presume that the limited number of Thompsons they had available, where in high demand at the time, not only for service use / training, but also being used for propaganda purposes, as we see in the many periodicals from that time, and the film you mention above.
As a footnote, while serving in different parts of Africa in the 1980's, we used Kalashnikovs on a number of occasions, but it was not until much later, as a civilian, that I learned that different countries manufactured AKs, with different markings on them. In the 1980's that was not a concern to us, we required weapons that operated and had a plentiful supply of suitable ammunition, and the markings on them were not even considered, by me anyway.
The request for early dispatch of two sample guns - which had to be Colt manufacture in February/March - was for among other reasons, the production of training manuals. Production and the writing of these pamphlets takes a while, no time to waste waiting for the first shipment of 180 (178) guns in a few months time. So it is not random that this Thompson in the photo with a horizontal forend is a Colt gun. I only wish the serial number could be read. Perhaps it is the same as the one in the Imperial War Museum? Could be another one that was located from within the UK, perhaps from Parker Hale who did advertise them in one of there catalogues or BSA who seemed to be the UK agent for some years, no way of knowing unless a high resolution copy of the photo turns up allowing the s/n to be read.
Arthur, I agree that some of the first 180 guns, in addition to the 2 that had to be Colts, could also have been Colt made guns as well. There is no definitive proof either way, unless a list of serial numbers of the first shipment turns up in an obscure file in the U.K. or U.S.A. Unlikely I know but these things do happen. I have found the list of 200 serial numbers for the second shipment of Colt made AR15's that the U.K. purchased in around 1963. The first shipment was for 100 rifles, I have not located this list - worst luck.
Anyway this has been a really good thread if you are into British service stuff. Personally, I still think and its only an opinion that the 178 guns of the 180 first shipment were Savage made, based mainly on the combined order of 750 guns being cancelled and then a fresh order for the same number being placed. Also the lack of surviving Colt guns with British ordnance markings, which in the 14,XXX serial number range is precisely one known example. As I said none of this is proof, just circumstantial.