Registering film for printing (optical) isn’t a mystery. It is simple, but has changed, with increasing complexity of mechanism making easier, more certain, faster production of picture.
At first, the method was: cut-n-butt. This was the way wash-off relief (dye transfer) was done. And done for 15 years.
Printers and animators had other means and methods. More about that in the “registration” posts.