Good Evening, Finny99,
When I worked with a St. Louis stock agency back in the 80's-90's, the split was 50-50 between the agency and the photographer. The photographer retained the copyright on all images and, indeed, returned dated transparencies from time to time. As far as I know that was standard practice for stock agencies. With the advent of digital imaging, I'm sure that the agencies have done a lot of adapting, but the basic principle of the photographer being the copyright-holder would, I assume, still be normal.
Konical