In doing this evaluation, you should know that Smith-Corona electrics after a certain time (I do not know when they developed this.) have rigged the repeating keys for small characters, period and comma and maybe dash/underscore, to strike with a lesser force precisely because they are smaller and more prone to damage the paper, etc. You can recognize these arrangements from underneath the keyboard by an extra bit in their linkages, near the key end. This will, of course result in a different sound when they hit the platen.

On a manual typewriter, the experienced typist could evolve a lighter technique on the small characters, but on an electric the typist cannot control that detail. I haven't looked, but I wonder whether any other manufacturers took such steps. I'm pretty sure IBM did in the Selectrics.

That Sears was made by Smith-Corona and Sears did spec some differences, conceivably including the spec for the platen rubber (but probably not that detail). The differences you are seeing and hearing are more likely due to the different experiences those two had over the last 50 years. One might have been left out in the sun all the time and the other mostly stored in a cool, dry, dark place, for example.

Let us know how it works out to stuff a damper into the hollow platen. But I think that S-C, being such an innovative engineering company, might have tried that and decided it wasn't worth it. Do let us know. Still, a damper won't protect the slugs from hitting a rock-hard platen, though. It would be best to get it recovered. I really like that Sears Medalist and it's worth treating well.