The 9mm followers are shorter than the 38 super followers on the STI mags. Hence the need for the spacers at the back. I've used 38 super/9mm mags on a Para/Rock Island platform and they work just fine. In the case of the 38/9 mags, it uses a 38 super follower so no spacer is needed but the 9mm rounds sit further back as they are shorter.