The CAA didn't like the perfectly acceptable solution of having the stick shaker operate at high speed (in place of RSB) as well as when approaching the stall.
The BA TriStars that went to the RAF (and became K1/KC1s) had RSB fitted (CAA certified). The Pan-Am ones (that became the C2/C2A) had the high speed shaker fitted (FAA certified). They were later brought into line and RSB fitted to the C2/C2A.
The CAA were concerned that at high altitude when a small margin existed between a Mach exceedance and the aerodynamic stall, the pilots would be confused as to the meaning of the stick shaker!
411A is right, my current 1970's -500 is just as advanced (and in some ways more so) than my last glass cockpit, HUD-equipped late 1990's designed piece of kit! It is a beautiful aircraft that rewards the skilled and saves the buffoons!