The edge of a metal file will easily cut a notch in an unhardened part but not in a hardened part. Email them for instructions covering how the parts are welded together. Hardened and mild steel parts are welded together similarly as cast iron parts are welded. Temperature is raised to a level based on the metals involved, the parts are welded then cooled slowly. Specific welding rods or filler wire are used. Close finish fit of the driveshaft and pinion gear shaft is essential for reasonable lifespan of the parts.
What happened to the lubrication of the splined sleeve? Did much of it migrate onto the silicone plug?