Size.
The main difference is that the TSW slide and frame were made as a matched set and have the same assembly number on each.
39 holds less rounds then the 59
The smith and Wesson model 10 has the K service frame.
The frame size of a Smith and Wesson model 66 is K.
take the grips off
The frame.
The Smith and Wesson K frame revolvers were made from 1946-1983.
Next smallest to the X frame.
It is a K-frame.
K frame
No, it is an L frame.
Standard Catalog of Smith and Wesson Firearms by Supica and Nahas History of Smith and Wesson by Jinks.