Next in line, my S&W 25-2.
PS: The Heavy Duty came about, due to a request from US police officers who fought increased violent crime during the Depression Era.
The regular M&P .38 Special, was perceived as not powerful enough (158 lead RN @750 fps) for use against gangsters who wore body armour, and the police bullets could not pierce auto bodies, of getaway cars. (not my words, this is well known to be the stated cause) S&W and Remington teamed up to provide a better launch vehicle/fodder, for increased velocity/pressure. S&W merely used the 44 frame, chambered in 38 Special (38-44, 38 Super Police, 38 High Velocity) using the hotter 38-44 ammunition produced specifically for this early N-frame sixgun. 1150 fps IIRC, using 158 weight?? Dimentionally 38 Special & 38-44 cartridges look exactly the same from the outside. It goes without saying, that 38-44 ammunition, should never be used in a less sturdy revolver, not designed to fire this higher pressure round. It was introduced in 1930 or 31. One could say this 38-44 Heavy Duty, was the "first magnum."
The next logical step up in velocity, was the 357 Magnum introduced in 1935, chambered in the S&W Registered Magnum revolver.
As well as the Colt New Service & Single Action Army sixguns.