The .351 caliber was mentioned in your previous topic “About the military Model of 1923”, in a reply I wrote, “I note that both the ‘Gazette des Armes’ article and ‘TUTB’, state that S/N 3075 was chambered for the Winchester .351 self-loading rifle cartridge”.
The Ultimate Thompson Book states on page 153 that “At least one, currently in a French military museum, is chambered for the Winchester .351 self-loading cartridge, which was used in Winchester self-loading sporting rifles by the French Air Service during WW1”.
Page 284 of the TUTB also mentions the French wished to test a Thompson in .351 calibre, as that cartridge already existed in the French military inventory, due to its use by WW1 balloon observers armed with the Winchester self-loading carbines. It goes on to state that two .351 caliber Thompsons were ordered by the French CEV, but only one was delivered in that caliber, together with a .45 calibre Thompson gun. The .351 calibre Thompson was tested again in 1926. After firing only 10 rounds the extractor broke and no spare parts had been ordered or shipped. So it was replaced with an extractor from a .45 model of 1921. Many failures ensued from this substitute extractor and testing ceased after 233 rounds as the.45 extractor became bent.
But now we have some confusion … The photograph in the French magazine “Gazette des Armes” you presented presents a model known as the "M1923 French Model .45 Remington-Thompson". However, according to the authors of this journal, they claim that this Thompson (S/N 3075) M1923 has .351 WSL…
It is possible that in 1924 John T. Thompson introduced the French Commission to this model TSMG .45 Remingdon-Thompson:
I suspect that a completely new receiver was required for the .351 WSL Thompson. The overall length of that cartridge is 1.906 inches, which is considerably longer than the .45 ACP (1.275), the .45 Remington-Thompson (1.385), or even the .30 Carbine (1.680). The standard dimension, front to back, of the mag well on a .45 ACP Thompson is 1.630 inches. As I recall, the .30 Carbine receiver example was modified from the standard to accommodate that longer cartridge, and the .351 WSL receiver would have had to be that much longer in the mag well area.
The probable requirement for a new receiver may be the reason that only one was produced for the French trials even though they ordered two. The fact that no spare parts were sent with the test rifle implies to me that it was a tool room creation to try to meet the interest/requirements of the French for this caliber. The magazine for the .30 Carbine Thompson appears to be an in-the-white one-off creation, and I suspect the same would have been true for the .351 WSL, at least for the trial gun. For many reasons, I seriously doubt that Auto-Ordnance would have modified their receiver and action to accommodate the use of the Winchester Model 1907 magazines.
If anyone is ever able to locate the .351 WSL Thompson in France, I am certain we would all love to see pics. Hopefully, dimensions could be measured and markings documented to clarify how this gun was actually made.