I hope you didn't pay much. You are also missing the blue ring (basically a rubber ring). It was the combination of the missing plate and missing blue ring that made the slipper what it was. Those two parts spread out the pressure on the slipper, instead of having all the pressure centered on the little spring. More even pressure made the spur gear run more true.
I'm sorry to say, but I think you are SOL. Both because of the missing parts and the fact that they are just two different designs of slipper, I don't see the strobe working on a 3903 trans.
Also, the strobe was an upgrade to the 3906 (you know this). The 3903 has a "revo spec" slipper that is just as good, or better than a strobe.
It's kind of a pointless upgrade, I'd actually consider it a downgrade.
Sorry man