Now that must have taken finesse.
Each frame of 35mm film has four sprocket holes in it, so if you randomly feed film into the projector you will have a 1 in 4 chance of the picture being framed correctly. I suppose he would probably have put framed leader on both ends of each reel to make sure he could nail it.
One of the challenges when a film breaks is that you need to make the splice on a frame line, and you must take out whole frames, or you will end up with a film that starts out in frame and then jumps off by one sprocket somewhere in the middle.
With traditional non-Cinemascope films there were big black bars between the frames, making it easy to find where to cut. With Cinemascope, they filled the entire frame with an anamorphic image, often times without even a hint of a frame line. So, if the film broke during a night scene, you might have to unreel twenty or thirty feet of film before you found a scene that showed the line well enough to frame correctly. Fortunately, once found, we had a little gadget that worked kind of like a surveyor’s measuring wheel that would roll along the film allowing us to find the correct spot closer to where the film broke.