The general answer is that transfer bars are not required in all revolvers because when there is greater freedom of form than that required by making a SAA replica, there are a variety of equally effective mechanical solutions to the same problem: that of insuring that the revolver will only fire when the trigger is pulled to rear and held there for the duration of the hammer fall.
One of the less common ones, but equally effective, was Hopkins & Allen's "Triple Action" mechanism, in which the hammer not only rotated in an arc, but also moved up and down. Pulling the trigger caused the hammer to move down so that the face of the hammer could strike the frame-mounted firing pin; releasing the trigger at any time caused the hammer to move up too high to hit the firing pin, and instead it struck the frame and rested there. As far I as know, only Hopkins & Allen and Llama of Spain used this system; I mentioned it recently in a thread where someone here was considering buying a Llama Comanche revolver.
I have in my possession a 6-shot H&A .32 Long Safety Police revolver, with the Triple Action mechanism, in which someone has ground off the projecting face of the hammer needed for the system to work. Now the firing pin protrudes into where the primer would be whenever the hammer is down, regardless of the trigger. I bought this gun off the Internet, and did not notice this flaw for months, because I did not take it to the range. Now I need to figure out what to do with it! An effective repair, or even replacing the hammer if an undamaged original became available, would probably cost more than the gun is worth. (It is not in pretty shape - I was attracted to it because it was a 32 Long.)