The first prototype Thompson had a higher rate of fire and it was purposely reduced. My brother has a 1921 and the 1928 models and a Youtube vid detailing the differences. https://www.youtube.com/watch?v=BsvWabSvi1I
You’re right. IIRC they also tried something called “dissimular metal adhesion” with early Thompsons to act as toggling to retard the rate of fire.
Or maybe there’s something to post #12.
Your brother is lucky both to have a 1921 model & to live where that’s allowed. I used to see `1928 overstamp’ on a lot of Thompsons for sale.
Of course, everything full auto has gone out of sight since 1986. I hope President Cruz will change that.
;^)