The 'Annihilator' prototypes (1919) were reputed to fire 1200 rounds per minute. The Colt made M1921 was advertised at a cyclic rate of 800, which was considered too fast by the Ordnance Department when tested for military use.

A quantity of Thompsons were ordered by the U.S. Navy as the 'Model of 1928', but with cyclic rate lowered to 600. This was achieved by replacing the 'actuator' (the reciprocating part with the charging knob) with one that was two ounces heavier and using lighter springs. The first orders of 1928's were filled by modifying existing Colt made examples which were 'overstamped' with an '8' over the final digit of the 'Model of 1921' designation.

Auto Ordnance, the firm marketing the Thompson, did not achieve the vast orders they sought for Thompsons until 1938 when the U.S. government ordered substantial quantities. After Colt made inventory was finally exhausted, the 'Model of 1928A1' were built by Auto Ordnance and by Savage Arms to the approved 1928 specification.

It was engineers at Savage who, in 1943, pushed for redesign of the Thompson to the much simplified M1 and M1A1 models which greatly reduced time and expense required for manufacture.

An excellent book on the Thompson is American Thunder by Frank Innamico which has since been revised as American Thunder 2. I have a copy of AT2, as well as a 1928A1 'parts kit' Thompson, one of the 'Lend Lease' Thompsons that were reputedly stored, unused, in Ukranian caves until the 1990s. They had initially been supplied to the USSR as standard equipment with M3 Stuart Tanks and White 'Half - Tracks'.

Here's a link to an article by the same author:

Thompson Article