John,
The one step film was made for a different camera but can be put into the SX70 by placing a thin piece of cardboard halfway into the opening and then sliding the onestep film over it and into the socket (the piece ejected by a new pack of film inserted in a camera is good). The one step has a tab that sticks down and you have to get it to clear the opening. Pull out the cardboard when the film is half-way in. The speed of the fim is different but I have removed the ND filter from the meter on my XS70 and it works fine with 600 film.