I can only take a few guesses at it's exact purpose.
Someone put it there as a filler or a stop for a handstop (guessing).
If it was a Winchester/Freeland rail I might guess it's to stop scratching the rail when moving accessories in the rail. But the Anschutz rail isn't as deep and most likely wouldn't fit above anything mounted in the rail.
I just glanced through the latest Champion SS catalog in case I'm out of date with this stuff. I am by the way, been a long time. But I didn't see anything like it.