The interstage between the S-IC and S-II stages are one of the two main differences between the two kits. The Airfix part is molded in two halves, resulting in detail loss along the joint. The Monogram version is pulled from a cylindrical mold, or at least it appears that way at first glance. Actually the part is slightly tapered, with the small end toward the bottom. The result is the black roll patterns, once painted, will give the illusion of not lining up when viewed at an angle. As this part is covered with simulate stringer detail, it is very difficult to fix or replace without doing the same thing to the other areas with stringer detail.
The Airfix part has the ullage motors molded on, while the other has them as separate parts. Separate motors allows the builder to simulate any of the 13 flight vehicles.