Here is the 33 foot diameter interstage between the S-IC and the S-II. The very bottom edge is the "field joint" where it connected to the forward skirt of the S-IC. Just above that is the "flight separation" joint, where the staging occured. Under that raised area would be the explosive charges to separate the two stages. In the center of the image are the fittings to attach the S-II ullage motors.