There was an M-frame.
S&W HE’s – Frame Sizes – S&W Hand Ejectors have letters designating their frame sizes. The "I" frame was the first, and was introduced with the S&W 32 HE First Model in 1896. It can be thought of as the typical frame size for a 6 shot .32 revolver. It is no longer produced, but with the introduction of the Chief’s Special in 38 Special caliber, the I frame was enlarged slightly to accept the longer cartridge and became the "J" frame, still immensely popular today for "pocket sized" back up and concealed carry revolvers. It may be thought of as the typical frame size for a 5 shot 38 Spl such as the "Chiefs Special". The "K" frame is the classic revolver of the 20th century. It was introduced in 1899 with the first S&W 38 Hand Ejector, also known as the .38 Military and Police (38 M&P). The model was aptly named as the K frame Smith became the basis for the most popular police issue revolvers of this century and for the famous Victory Models, issued to the Military during WWII. It may be thought of as the typical frame for a 6 shot 38 Special. The big "N" frame was introduced in 1908 for the famous S&W Triplelock 44 revolver. It has continued to become the basis for S&W’s Model 29 44 Magnum series of revolvers and could be considered the typical frame for a 44 or 45 caliber 6 shot revolver. The tiny M frame was used only for the 7 shot .22 Ladysmith revolver of the early 20th century.