By the way, there were 4 different versions of SAT Vol. I No. 21. They are as follows:
1940: "Thompson Sub-Machine-Gun"
1942: "The Thompson Machine Carbine"
1944: " The Machine Carbine" (Covered both the Thompson and the Sten)
1955: "The Machine Carbine" (Covered both the Thompson and the Sten, with updates since 1944 added in)
Here is what I found that I think folks on this board may find interesting:
This is from the 1940 edition, showing an M1928 with a horizontal foregrip, and the sling installed under the foregrip. Seems normal enough...

The 1942 edition shows 3 different front sling attachment locations, and mentions 4 different locations, with different grips. (Two illustrations with the vertical foregrip are noted as incorrect.) The first illustration (Fig. 1) shows an M1928 with horizontal foregrip, and the sling attached on the left hand side, and the top of the buttstock.

The second illustration (Fig. 2) shows the M1928 with horizontal foregrip with the sling attached on the bottom:

The third illustration (Fig. 3) shows an M1928 with a vertical foregrip with the sling attached on the right side. It is noted that the reader should refer to figure 1 for correct sling swivel location, not as in the illustration.

The fourth illustatration (Fig. 4) is similar to Fig. 3, and shows an M1928 with a vertical foregrip with the sling attached on the right side, with the same reference note as in Figure 3.

If you look on page 83 of ATII, there is a picture of a British M1928 with the front swivel attached on the left side of a vertical foregrip, as referenced in writing in Fig. 1 of the 1942 edition British Thompson manual.
If you look on page 81 of ATII, there is a picture of a British M1928 with the front swivel attached on the right side of the vertical foregrip. This was supposed to be incorrect placement according to the 1942 manual, and yet was still pictured in 2 line drawings, as you saw previously in my pictures.
Anyway, my point is that front sling placement on the Thompson did not seem to be standardized in the British Army during World War II, and their manuals (especially the 1942 edition) seemed to document confusion in this matter.
Any thoughts on the subject?
Thanks!
David Albert
dalbert@sturmgewehr.com