Just checked and you're right. Early four digit guns and 1928. No 1921s...in sight.
Pam is short for Pamphlet, training. The book.
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Most military Model 1921 in the US were converted to the US Navy Model 1928 which had the 4oz weight bolted to
the actuator to slow down the rate of fire plus being over stamped with the number 8 over the 1
Some Colt guns were in the first orders, some of them used but reconditioned.
There are early photos of Germans in France guarding the railroad cars holding Thompson smg's taken around 1940-41.
The Thompson's shown with the Germans have finned barrels but without the Cutts compensator, this would indicate to
me that these are Colt 1921 or over stamp 21/8 models. All the Savage and Auto Ord Model 1928A1 Thompsons had the
Cutts compensator while the Colt 1921 models could be with the Cutts compensator or without it. France did receive some early Colt 1921 TSMG's